[PATCH v9 00/11] Dynamic PAMT

[PATCH v9 00/11] Dynamic PAMT

[Rick Edgecombe]

Hi,

This another revision of Dynamic PAMT TDX series. Like v7 and v8, it is 
hopefully the last. Last time it got two non-functional nits from Sashiko 
and two false positives. After some offlist discussion with Dave, I've
applied the nits and rebased on v7.2-rc6. There were no conflicts.
Everything else is the same.

Background
==========
Dynamic PAMT is a TDX feature that allows saving memory by allocating some 
of its page tracking metadata dynamically, instead of statically at boot. 
These static allocations take roughly 0.4% of system memory. The savings 
are variable depending on system and TDX usage, but could be up to 100x. 
For more Dynamic PAMT background, please refer to [1]. For more analysis 
of the savings in different scenarios, see the v6 coverleter[0].

It occurred to me that since the Dynamic PAMT effort began, RAM has become
much more expensive. Consequently, this feature is even more valuable now.
It would be good to enable it for TDX users.

Optimization patch
==================
The past two revision I mentioned some considerations for whether to 
include the get/put optimization patch. In v6 Dave had asked whether the 
"x86/virt/tdx: Optimize tdx_pamt_get/put()" was really needed. Later we 
discussed offline to keep the patch for the sake of maintaining 
performance and being kind to KVM's efforts to fault under a shared lock. 
However, now that the feature requires an opt-in, it could be for limited 
use and not disturb any kernel upgraders. Then the optimization patch 
actually does become more optional. So it was moved to the end.

On v7 that patch picked up a RB from Nikolay. I think it is still optional,
but I'd lean towards recommending it. But I'm ok either way, if we want to 
discuss it more without holding up the basic support.

Base
====
This is based on v7.2-rc6. A branch can be found here: [2].

Testing
=======
This last version was tested in the usual suite, and also with the 
optimization patch removed. The validation person who has worked on this 
series added his Tested-by offlist. He has a special dynamic PAMT specific 
test suite. Since there were not any changes which would be expected to 
have functional impact I just did some regression testing with our standard 
automated testing for this one. I left his Tested-by tags per his 
preference.


[0] https://lore.kernel.org/lkml/20260526023515.288829-1-rick.p.edgecombe@intel.com/
[1] https://lore.kernel.org/lkml/20250918232224.2202592-1-rick.p.edgecombe@intel.com/
[2] https://github.com/intel-staging/tdx/tree/dpamt_v9


Kiryl Shutsemau (9):
  x86/virt/tdx: Allocate page bitmap for Dynamic PAMT
  x86/virt/tdx: Add tdx_alloc/free_control_page() helpers
  x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory
  x86/virt/tdx: Handle multiple callers in tdx_pamt_get/put()
  KVM: TDX: Allocate PAMT memory for TD and vCPU control structures
  KVM: TDX: Get/put PAMT pages when (un)mapping private memory
  x86/virt/tdx: Enable Dynamic PAMT
  Documentation/x86: Add documentation for TDX's Dynamic PAMT
  x86/virt/tdx: Optimize tdx_pamt_get/put()

Rick Edgecombe (2):
  x86/virt/tdx: Simplify PAMT layout calculation
  x86/tdx: Add APIs to support Dynamic PAMT ops from KVM's fault path

 .../admin-guide/kernel-parameters.txt         |  10 +
 Documentation/arch/x86/tdx.rst                |  28 ++
 arch/x86/include/asm/kvm-x86-ops.h            |   1 +
 arch/x86/include/asm/kvm_host.h               |   1 +
 arch/x86/include/asm/tdx.h                    |  26 +
 arch/x86/include/asm/tdx_global_metadata.h    |   3 +
 arch/x86/kvm/mmu/mmu.c                        |   4 +
 arch/x86/kvm/vmx/tdx.c                        |  99 ++--
 arch/x86/kvm/vmx/tdx.h                        |   2 +
 arch/x86/virt/vmx/tdx/tdx.c                   | 452 +++++++++++++++---
 arch/x86/virt/vmx/tdx/tdx.h                   |   2 +
 arch/x86/virt/vmx/tdx/tdx_global_metadata.c   |  23 +-
 12 files changed, 559 insertions(+), 92 deletions(-)

-- 
2.54.0

[PATCH v9 01/11] x86/virt/tdx: Simplify PAMT layout calculation

[Rick Edgecombe]

For each memory region that the TDX module might use (called TDMR), three
separate traditional PAMT allocations are needed. There is one for each
supported page size (1GB, 2MB, 4KB). These store information on each page
in the TDMR. In Linux, they are allocated out of one physically contiguous
block, in order to more efficiently use some internal TDX module
bookkeeping resources. So some simple math is needed to break the single
large allocation into three smaller allocations for each page size.

There are some commonalities in the math needed to calculate the base and
size for each smaller allocation, and so an effort was made to share logic
across the three. Unfortunately doing this turned out unnaturally tortured,
with a loop iterating over the three page sizes, only to call into a
function with case statements for each page size. In the future Dynamic
PAMT will add more logic that is special to the 4KB page size, making the
benefit of the math sharing even more questionable.

Three is not a very high number, so get rid of the loop and just duplicate
the small calculation three times. In doing so, setup for future Dynamic
PAMT changes.

Since the loop that iterates over it is gone, further simplify the code by
dropping the array of intermediate size and base storage. Just store the
values to their final locations.

AI was used under supervision to collect/apply feedback, review code and
workshop logs.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
---
v8:
 - Fix PAMT capitalization in comment (Sohil)
 - Delete stale info in log, that Chao mentioned last time (Binbin)

v7:
 - Remove accidentital whitespace changes (Kiryl)
 - Drop stale sentence in log (Chao)
 - Better patch subject (Yan)
 - Drop Assisted-by tag and cover AI use in log (Dave)
---
 arch/x86/virt/vmx/tdx/tdx.c | 90 ++++++++++++-------------------------
 1 file changed, 28 insertions(+), 62 deletions(-)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 42df8ea464c47..e00a6c8eda1b9 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -514,31 +514,21 @@ static __init int fill_out_tdmrs(struct list_head *tmb_list,
  * Calculate PAMT size given a TDMR and a page size.  The returned
  * PAMT size is always aligned up to 4K page boundary.
  */
-static __init unsigned long tdmr_get_pamt_sz(struct tdmr_info *tdmr, int pgsz,
-					     u16 pamt_entry_size)
+static __init unsigned long tdmr_get_pamt_sz(struct tdmr_info *tdmr, int pgsz)
 {
 	unsigned long pamt_sz, nr_pamt_entries;
+	const int tdx_pg_size_shift[TDX_PS_NR] = { PAGE_SHIFT, PMD_SHIFT, PUD_SHIFT };
+	const u16 pamt_entry_size[TDX_PS_NR] = {
+		tdx_sysinfo.tdmr.pamt_4k_entry_size,
+		tdx_sysinfo.tdmr.pamt_2m_entry_size,
+		tdx_sysinfo.tdmr.pamt_1g_entry_size,
+	};
 
-	switch (pgsz) {
-	case TDX_PS_4K:
-		nr_pamt_entries = tdmr->size >> PAGE_SHIFT;
-		break;
-	case TDX_PS_2M:
-		nr_pamt_entries = tdmr->size >> PMD_SHIFT;
-		break;
-	case TDX_PS_1G:
-		nr_pamt_entries = tdmr->size >> PUD_SHIFT;
-		break;
-	default:
-		WARN_ON_ONCE(1);
-		return 0;
-	}
+	nr_pamt_entries = tdmr->size >> tdx_pg_size_shift[pgsz];
+	pamt_sz = nr_pamt_entries * pamt_entry_size[pgsz];
 
-	pamt_sz = nr_pamt_entries * pamt_entry_size;
 	/* TDX requires PAMT size must be 4K aligned */
-	pamt_sz = ALIGN(pamt_sz, PAGE_SIZE);
-
-	return pamt_sz;
+	return PAGE_ALIGN(pamt_sz);
 }
 
 /*
@@ -576,15 +566,11 @@ static __init int tdmr_get_nid(struct tdmr_info *tdmr, struct list_head *tmb_lis
  * within @tdmr, and set up PAMTs for @tdmr.
  */
 static __init int tdmr_set_up_pamt(struct tdmr_info *tdmr,
-				   struct list_head *tmb_list,
-				   u16 pamt_entry_size[])
+				   struct list_head *tmb_list)
 {
-	unsigned long pamt_base[TDX_PS_NR];
-	unsigned long pamt_size[TDX_PS_NR];
-	unsigned long tdmr_pamt_base;
 	unsigned long tdmr_pamt_size;
 	struct page *pamt;
-	int pgsz, nid;
+	int nid;
 
 	nid = tdmr_get_nid(tdmr, tmb_list);
 
@@ -592,12 +578,10 @@ static __init int tdmr_set_up_pamt(struct tdmr_info *tdmr,
 	 * Calculate the PAMT size for each TDX supported page size
 	 * and the total PAMT size.
 	 */
-	tdmr_pamt_size = 0;
-	for (pgsz = TDX_PS_4K; pgsz < TDX_PS_NR; pgsz++) {
-		pamt_size[pgsz] = tdmr_get_pamt_sz(tdmr, pgsz,
-					pamt_entry_size[pgsz]);
-		tdmr_pamt_size += pamt_size[pgsz];
-	}
+	tdmr->pamt_4k_size = tdmr_get_pamt_sz(tdmr, TDX_PS_4K);
+	tdmr->pamt_2m_size = tdmr_get_pamt_sz(tdmr, TDX_PS_2M);
+	tdmr->pamt_1g_size = tdmr_get_pamt_sz(tdmr, TDX_PS_1G);
+	tdmr_pamt_size = tdmr->pamt_4k_size + tdmr->pamt_2m_size + tdmr->pamt_1g_size;
 
 	/*
 	 * Allocate one chunk of physically contiguous memory for all
@@ -606,25 +590,17 @@ static __init int tdmr_set_up_pamt(struct tdmr_info *tdmr,
 	 */
 	pamt = alloc_contig_pages(tdmr_pamt_size >> PAGE_SHIFT, GFP_KERNEL,
 			nid, &node_online_map);
+
+	/*
+	 * tdmr->pamt_4k_base is still zero so the error
+	 * path of the caller will skip freeing the PAMT.
+	 */
 	if (!pamt)
 		return -ENOMEM;
 
-	/*
-	 * Break the contiguous allocation back up into the
-	 * individual PAMTs for each page size.
-	 */
-	tdmr_pamt_base = page_to_pfn(pamt) << PAGE_SHIFT;
-	for (pgsz = TDX_PS_4K; pgsz < TDX_PS_NR; pgsz++) {
-		pamt_base[pgsz] = tdmr_pamt_base;
-		tdmr_pamt_base += pamt_size[pgsz];
-	}
-
-	tdmr->pamt_4k_base = pamt_base[TDX_PS_4K];
-	tdmr->pamt_4k_size = pamt_size[TDX_PS_4K];
-	tdmr->pamt_2m_base = pamt_base[TDX_PS_2M];
-	tdmr->pamt_2m_size = pamt_size[TDX_PS_2M];
-	tdmr->pamt_1g_base = pamt_base[TDX_PS_1G];
-	tdmr->pamt_1g_size = pamt_size[TDX_PS_1G];
+	tdmr->pamt_4k_base = page_to_phys(pamt);
+	tdmr->pamt_2m_base = tdmr->pamt_4k_base + tdmr->pamt_4k_size;
+	tdmr->pamt_1g_base = tdmr->pamt_2m_base + tdmr->pamt_2m_size;
 
 	return 0;
 }
@@ -655,10 +631,7 @@ static __init void tdmr_do_pamt_func(struct tdmr_info *tdmr,
 	tdmr_get_pamt(tdmr, &pamt_base, &pamt_size);
 
 	/* Do nothing if PAMT hasn't been allocated for this TDMR */
-	if (!pamt_size)
-		return;
-
-	if (WARN_ON_ONCE(!pamt_base))
+	if (!pamt_base)
 		return;
 
 	pamt_func(pamt_base, pamt_size);
@@ -684,14 +657,12 @@ static __init void tdmrs_free_pamt_all(struct tdmr_info_list *tdmr_list)
 
 /* Allocate and set up PAMTs for all TDMRs */
 static __init int tdmrs_set_up_pamt_all(struct tdmr_info_list *tdmr_list,
-					struct list_head *tmb_list,
-					u16 pamt_entry_size[])
+				 struct list_head *tmb_list)
 {
 	int i, ret = 0;
 
 	for (i = 0; i < tdmr_list->nr_consumed_tdmrs; i++) {
-		ret = tdmr_set_up_pamt(tdmr_entry(tdmr_list, i), tmb_list,
-				pamt_entry_size);
+		ret = tdmr_set_up_pamt(tdmr_entry(tdmr_list, i), tmb_list);
 		if (ret)
 			goto err;
 	}
@@ -968,18 +939,13 @@ static __init int construct_tdmrs(struct list_head *tmb_list,
 				  struct tdmr_info_list *tdmr_list,
 				  struct tdx_sys_info_tdmr *sysinfo_tdmr)
 {
-	u16 pamt_entry_size[TDX_PS_NR] = {
-		sysinfo_tdmr->pamt_4k_entry_size,
-		sysinfo_tdmr->pamt_2m_entry_size,
-		sysinfo_tdmr->pamt_1g_entry_size,
-	};
 	int ret;
 
 	ret = fill_out_tdmrs(tmb_list, tdmr_list);
 	if (ret)
 		return ret;
 
-	ret = tdmrs_set_up_pamt_all(tdmr_list, tmb_list, pamt_entry_size);
+	ret = tdmrs_set_up_pamt_all(tdmr_list, tmb_list);
 	if (ret)
 		return ret;
 
-- 
2.54.0

Re: [PATCH v9 01/11] x86/virt/tdx: Simplify PAMT layout calculation

[Dave Hansen]

On 8/5/26 19:08, Rick Edgecombe wrote:
> Three is not a very high number, so get rid of the loop and just duplicate
> the small calculation three times. In doing so, setup for future Dynamic
> PAMT changes.

Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

[PATCH v9 02/11] x86/virt/tdx: Allocate page bitmap for Dynamic PAMT

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

The TDX Physical Address Metadata Table (PAMT) holds data about the
physical memory used by TDX, and must be allocated by the kernel during
TDX module initialization.

The exact size of the required PAMT memory is determined by the TDX module
and may vary between TDX module versions. Currently it is approximately
0.4% of the system memory. This is a significant commitment, especially if
it is not known upfront whether the machine will run any TDX guests.

Each memory region that the TDX module might use needs three separate PAMT
allocations. One for each supported page size (1GB, 2MB, 4KB). The
TDX module supports a new feature designed to reduce PAMT overhead called
Dynamic PAMT. Under Dynamic PAMT the 4KB level is allocated dynamically
during runtime, while the 1GB and 2MB levels remain allocated on TDX
module initialization.

However, in the details, Dynamic PAMT still needs some smaller per 4KB
page scoped data (currently it is 1 bit per page). The TDX module exposes
the number of bits as a separate piece of metadata than the 4KB static
allocation for normal PAMT. Although the size is enumerated differently,
it is handed to the TDX module in the same way the 4KB page size PAMT
allocation is for normal PAMT.

Begin to implement Dynamic PAMT in the kernel by reading the bits-per-page
needed for Dynamic PAMT. Calculate the size needed for the bitmap,
and use it instead of the 4KB size determined for normal PAMT, in the case
of Dynamic PAMT.

The existing metadata reading code was generated by a script, but the
current plan is to stop generating this code, as the script has continued
to need adjustments. So add manually written code and adjust the comment
about it being autogenerated to be more generic. Start to adopt a more
normal kernel code style without the ternary statements and if
conditionals assignments that the auto generated code has.

AI was used under supervision to collect/apply feedback, review code and
workshop logs.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Vishal Annapurve <vannapurve@google.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
---
v9:
 - Switch to size byte size hint in metadata field id (Sashiko)

v7:
 - Re-order pamt size calculations for greater readability (Kiryl)
 - Move comment to its own line in tdx_supports_dynamic_pamt() (Yan,
   Sohil)
 - Log tweak (Sohil)
 - Make comment in metadata reading more appropriate (Sohil)
 - Drop Assisted-by tag and cover AI use in log (Dave)
 - Move tdx_supports_dynamic_pamt() to not static inline to reduce
   churn in later changes

v6:
 - Improve comment (Binbin)
 - Log tweaks
 - Mark tdmr_get_pamt_bitmap_sz() __init in response to upstream
   changes
 - Switch to more normal kernel code style, even though it differs from
   the existing auto generated code.
---
 arch/x86/include/asm/tdx.h                  |  2 ++
 arch/x86/include/asm/tdx_global_metadata.h  |  3 +++
 arch/x86/virt/vmx/tdx/tdx.c                 | 29 +++++++++++++++++++--
 arch/x86/virt/vmx/tdx/tdx_global_metadata.c | 23 +++++++++++++++-
 4 files changed, 54 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 89e97d5761d89..d414064436221 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -118,6 +118,8 @@ static inline bool tdx_supports_runtime_update(const struct tdx_sys_info *sysinf
 	return sysinfo->features.tdx_features0 & TDX_FEATURES0_TD_PRESERVING;
 }
 
+bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo);
+
 int tdx_guest_keyid_alloc(void);
 u32 tdx_get_nr_guest_keyids(void);
 void tdx_guest_keyid_free(unsigned int keyid);
diff --git a/arch/x86/include/asm/tdx_global_metadata.h b/arch/x86/include/asm/tdx_global_metadata.h
index 41150d546589c..2a42551fc33cd 100644
--- a/arch/x86/include/asm/tdx_global_metadata.h
+++ b/arch/x86/include/asm/tdx_global_metadata.h
@@ -21,6 +21,9 @@ struct tdx_sys_info_tdmr {
 	u16 pamt_4k_entry_size;
 	u16 pamt_2m_entry_size;
 	u16 pamt_1g_entry_size;
+
+	/* Optional metadata, if Dynamic PAMT is supported */
+	u8  pamt_page_bitmap_entry_bits;
 };
 
 struct tdx_sys_info_td_ctrl {
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index e00a6c8eda1b9..9edad6440080e 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -510,6 +510,18 @@ static __init int fill_out_tdmrs(struct list_head *tmb_list,
 	return 0;
 }
 
+static __init unsigned long tdmr_get_pamt_bitmap_sz(struct tdmr_info *tdmr)
+{
+	unsigned long pamt_sz, nr_pamt_entries;
+	int bits_per_entry;
+
+	bits_per_entry = tdx_sysinfo.tdmr.pamt_page_bitmap_entry_bits;
+	nr_pamt_entries = tdmr->size >> PAGE_SHIFT;
+	pamt_sz = DIV_ROUND_UP(nr_pamt_entries * bits_per_entry, BITS_PER_BYTE);
+
+	return PAGE_ALIGN(pamt_sz);
+}
+
 /*
  * Calculate PAMT size given a TDMR and a page size.  The returned
  * PAMT size is always aligned up to 4K page boundary.
@@ -578,9 +590,16 @@ static __init int tdmr_set_up_pamt(struct tdmr_info *tdmr,
 	 * Calculate the PAMT size for each TDX supported page size
 	 * and the total PAMT size.
 	 */
-	tdmr->pamt_4k_size = tdmr_get_pamt_sz(tdmr, TDX_PS_4K);
-	tdmr->pamt_2m_size = tdmr_get_pamt_sz(tdmr, TDX_PS_2M);
 	tdmr->pamt_1g_size = tdmr_get_pamt_sz(tdmr, TDX_PS_1G);
+	tdmr->pamt_2m_size = tdmr_get_pamt_sz(tdmr, TDX_PS_2M);
+
+	if (tdx_supports_dynamic_pamt(&tdx_sysinfo)) {
+		/* With Dynamic PAMT, PAMT_4K is replaced with a bitmap */
+		tdmr->pamt_4k_size = tdmr_get_pamt_bitmap_sz(tdmr);
+	} else {
+		tdmr->pamt_4k_size = tdmr_get_pamt_sz(tdmr, TDX_PS_4K);
+	}
+
 	tdmr_pamt_size = tdmr->pamt_4k_size + tdmr->pamt_2m_size + tdmr->pamt_1g_size;
 
 	/*
@@ -1969,6 +1988,12 @@ u64 tdh_phymem_page_wbinvd_hkid(u64 hkid, kvm_pfn_t pfn)
 }
 EXPORT_SYMBOL_FOR_KVM(tdh_phymem_page_wbinvd_hkid);
 
+bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo)
+{
+	/* To be enabled when kernel is ready. */
+	return false;
+}
+
 void tdx_sys_disable(void)
 {
 	struct tdx_module_args args = {};
diff --git a/arch/x86/virt/vmx/tdx/tdx_global_metadata.c b/arch/x86/virt/vmx/tdx/tdx_global_metadata.c
index e49c300f23d43..93b1f26252c63 100644
--- a/arch/x86/virt/vmx/tdx/tdx_global_metadata.c
+++ b/arch/x86/virt/vmx/tdx/tdx_global_metadata.c
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- * Automatically generated functions to read TDX global metadata.
+ * Functions to read TDX global metadata.
  *
  * This file doesn't compile on its own as it lacks of inclusion
  * of SEAMCALL wrapper primitive which reads global metadata.
@@ -33,6 +33,18 @@ static __init int get_tdx_sys_info_features(struct tdx_sys_info_features *sysinf
 	return ret;
 }
 
+static __init int get_tdx_sys_info_tdmr_dpamt(struct tdx_sys_info_tdmr *sysinfo_tdmr)
+{
+	int ret;
+	u64 val;
+
+	ret = read_sys_metadata_field(0x9100000000000013, &val);
+	if (!ret)
+		sysinfo_tdmr->pamt_page_bitmap_entry_bits = val;
+
+	return ret;
+}
+
 static __init int get_tdx_sys_info_tdmr(struct tdx_sys_info_tdmr *sysinfo_tdmr)
 {
 	int ret = 0;
@@ -129,5 +141,14 @@ static __init int get_tdx_sys_info(struct tdx_sys_info *sysinfo)
 	ret = ret ?: get_tdx_sys_info_td_ctrl(&sysinfo->td_ctrl);
 	ret = ret ?: get_tdx_sys_info_td_conf(&sysinfo->td_conf);
 
+	/*
+	 * The kernel supports using TDX without Dynamic PAMT, so
+	 * avoid reporting failure if it's not supported. Don't try
+	 * to support buggy TDX modules that advertise Dynamic PAMT
+	 * but don't expose the metadata.
+	 */
+	if (!ret && tdx_supports_dynamic_pamt(sysinfo))
+		ret = get_tdx_sys_info_tdmr_dpamt(&sysinfo->tdmr);
+
 	return ret;
 }
-- 
2.54.0

Re: [PATCH v9 02/11] x86/virt/tdx: Allocate page bitmap for Dynamic PAMT

[Dave Hansen]

On 8/5/26 19:08, Rick Edgecombe wrote:
> Begin to implement Dynamic PAMT in the kernel by reading the bits-per-page
> needed for Dynamic PAMT. Calculate the size needed for the bitmap,
> and use it instead of the 4KB size determined for normal PAMT, in the case
> of Dynamic PAMT.

Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

[PATCH v9 03/11] x86/virt/tdx: Add tdx_alloc/free_control_page() helpers

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Add helpers to use when allocating or preparing pages that are handed to
the TDX module for use as control/S-EPT pages, and thus need Dynamic PAMT
adjustments.

The TDX module tracks some state for each page of physical memory that it
might use. It calls this state the PAMT. It includes separate state for
each page size a physical page could be utilized at within the TDX module
(1GB, 2MB, 4KB). In Dynamic PAMT, only the 4KB page size state is
allocated dynamically.

KVM will need to hand pages to the TDX module that it will use at 4KB
granularity. So these pages will need Dynamic PAMT backing added before
they are used by the TDX module, and removed afterwards.

Add tdx_alloc_control_page() and tdx_free_control_page() to handle both
page allocation and Dynamic PAMT installation. Make them behave like
normal alloc/free functions where allocation can fail in the case of no
memory, but free (with any necessary Dynamic PAMT release) always
succeeds. Do this so they can support the existing TDX flows that require
teardowns to succeed.

Also create tdx_pamt_get/put() to handle installing Dynamic PAMT 4KB
backing for pages that are already allocated (such as KVM's use of S-EPT
page tables or guest private memory). Have them take a pfn instead of a
struct page, as future changes will want to use these helpers for guest
pages which are tracked by PFN.

Don't CLFLUSH the Dynamic PAMT pages handed to the TDX module, as is done
for some other SEAMCALLs, as the TDX docs specify that this is only
needed on "TD private memory or TD control structure page".

Since these allocations will be easily user triggerable, account the
memory.

Only one pair of Dynamic PAMT pages is required for each 2MB-aligned
physical region, so multiple callers could trip over each other if they
try to manage the shared backing for two separate 4KB pages contained in
one. To build the logic up iteratively, don't do anything to handle pages
from the same 2MB region yet. Functionality to handle this will be added
before Dynamic PAMT can be enabled.

AI was used under supervision to collect/apply feedback, split patches,
review code and workshop logs.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
---
v9:
 - Address about how the log doesn't fully cover the limitations at this
   stage. (Sashiko)

v7:
 - Comment improvements (Chao)
 - Drop unneeded addition of mm.h header include (Binbin)
 - Log clarity, code style nits (Sohil)
 - Drop Assisted-by tag and cover AI use in log (Dave)

v6:
The major change was to split out the concurrency stuff into a future
patch. It makes it easier to explain in the log. This one is the basic
functionality. Then the simple version of the concurrency and why in the
next patch. Also, to get rid of the dynamically sized DPAMT backing
support which was not based on a formal spec.

Details:
 - Split out concurrency stuff into next patch because the log was too long
 - Switch to fixed size pamt page arrays (Nikolay)
 - Rename tdx_alloc_page()/tdx_free_page() to tdx_alloc_control_page()/
   tdx_free_control_page() to reflect control/S-EPT purpose (Sean)
 - Take gfp from the caller in tdx_alloc_control_page() (Sean)
 - Narrow external API: make tdx_pamt_get()/tdx_pamt_put() static and
   export only tdx_alloc_control_page()/tdx_free_control_page() (note:
   dropped inline helpers since the discussion on Sean's series resulted
   in them not being needed)
 - Switch EXPORT_SYMBOL_GPL to EXPORT_SYMBOL_FOR_KVM (Sean)
 - Use WARN_ON_ONCE() instead of pr_err() for TDX module failures (Sean)
 - Fold alloc_pamt_array()/free_pamt_array() helpers back in and fix the
   error-unwind index bug (dpamt_pages[i] -> [j])
 - Adjustments after struct page->pfn
 - Adjustments from dropping error helper patches
 - Make the free error paths more normal
 - Drop gfp_t arg in tdx_alloc_control_page(). In the Sean mega v5, it
   was really needed because the kvm_mmu_memory_cache had a gfp_t it
   needed something to do with. But this was still weird because that
   version didn't handle allocating the DPAMT pages as the gfp_t. And in
   the end all the callers pass GFP_KERNEL_ACCOUNT. So just drop the arg.
 - Log tweaks
---
 arch/x86/include/asm/tdx.h  |   6 ++
 arch/x86/virt/vmx/tdx/tdx.c | 163 ++++++++++++++++++++++++++++++++++++
 arch/x86/virt/vmx/tdx/tdx.h |   2 +
 3 files changed, 171 insertions(+)

diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index d414064436221..86bf37b15c705 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -126,6 +126,12 @@ void tdx_guest_keyid_free(unsigned int keyid);
 
 void tdx_quirk_reset_paddr(unsigned long base, unsigned long size);
 
+/* Number of PAMT pages to be provided to TDX module per 2MB region of PA */
+#define TDX_DPAMT_ENTRY_PAGE_CNT 2
+
+struct page *tdx_alloc_control_page(void);
+void tdx_free_control_page(struct page *page);
+
 struct tdx_td {
 	/* TD root structure: */
 	struct page *tdr_page;
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 9edad6440080e..4d3427b8a5b71 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -1994,6 +1994,169 @@ bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo)
 	return false;
 }
 
+static int alloc_pamt_array(struct page **pamt_pages)
+{
+	int i, j;
+
+	for (i = 0; i < TDX_DPAMT_ENTRY_PAGE_CNT; i++) {
+		pamt_pages[i] = alloc_page(GFP_KERNEL_ACCOUNT);
+		if (!pamt_pages[i])
+			goto err;
+	}
+
+	return 0;
+
+err:
+	for (j = 0; j < i; j++)
+		__free_page(pamt_pages[j]);
+
+	return -ENOMEM;
+}
+
+static void free_pamt_array(struct page **pamt_pages)
+{
+	for (int i = 0; i < TDX_DPAMT_ENTRY_PAGE_CNT; i++) {
+		/*
+		 * Reset pages unconditionally to cover cases
+		 * where they were passed to the TDX module.
+		 */
+		tdx_quirk_reset_paddr(page_to_phys(pamt_pages[i]), PAGE_SIZE);
+
+		__free_page(pamt_pages[i]);
+	}
+}
+
+/*
+ * Calculate the arg needed for operating on the DPAMT backing for
+ * a given 4KB page.
+ */
+static u64 pamt_2mb_arg(kvm_pfn_t pfn)
+{
+	/* Arg value will specify a 2MB region of physical address space. */
+	unsigned long hpa_2mb = ALIGN_DOWN(pfn << PAGE_SHIFT, PMD_SIZE);
+
+	return hpa_2mb | TDX_PS_2M;
+}
+
+/* Add PAMT backing for the 2MB region surrounding the given pfn. */
+static u64 tdh_phymem_pamt_add(kvm_pfn_t pfn, struct page **pamt_pages)
+{
+	struct tdx_module_args args = {
+		.rcx = pamt_2mb_arg(pfn),
+		.rdx = page_to_phys(pamt_pages[0]),
+		.r8 = page_to_phys(pamt_pages[1]),
+	};
+
+	return seamcall(TDH_PHYMEM_PAMT_ADD, &args);
+}
+
+/* Remove PAMT backing for the 2MB region surrounding the given pfn. */
+static u64 tdh_phymem_pamt_remove(kvm_pfn_t pfn, struct page **pamt_pages)
+{
+	struct tdx_module_args args = {
+		.rcx = pamt_2mb_arg(pfn),
+	};
+	u64 ret;
+
+	ret = seamcall_ret(TDH_PHYMEM_PAMT_REMOVE, &args);
+	if (ret)
+		return ret;
+
+	/* Copy PAMT pages out of the struct per the TDX ABI */
+	pamt_pages[0] = phys_to_page(args.rdx);
+	pamt_pages[1] = phys_to_page(args.r8);
+
+	return 0;
+}
+
+/* Allocate PAMT memory for the given page */
+static int tdx_pamt_get(kvm_pfn_t pfn)
+{
+	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT];
+	u64 tdx_status;
+	int ret;
+
+	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
+		return 0;
+
+	ret = alloc_pamt_array(pamt_pages);
+	if (ret)
+		return ret;
+
+	tdx_status = tdh_phymem_pamt_add(pfn, pamt_pages);
+	if (tdx_status != TDX_SUCCESS) {
+		ret = -EIO;
+		goto out_free;
+	}
+
+	return 0;
+
+out_free:
+	free_pamt_array(pamt_pages);
+
+	return ret;
+}
+
+/* Free PAMT memory for the given page */
+static void tdx_pamt_put(kvm_pfn_t pfn)
+{
+	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT] = {};
+	u64 tdx_status;
+
+	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
+		return;
+
+	tdx_status = tdh_phymem_pamt_remove(pfn, pamt_pages);
+
+	/*
+	 * Don't free pamt_pages as it could hold garbage when
+	 * tdh_phymem_pamt_remove() fails.  Don't panic/BUG_ON(), as
+	 * there is no risk of data corruption, but do yell loudly as
+	 * failure indicates a kernel bug, memory is being leaked, and
+	 * the dangling PAMT entry may cause future operations to fail.
+	 */
+	if (WARN_ON_ONCE(tdx_status != TDX_SUCCESS))
+		return;
+
+	free_pamt_array(pamt_pages);
+}
+
+/*
+ * Return a page that can be gifted to the TDX-Module for use as a "control"
+ * page, i.e. pages that are used for control structures for a given TDX
+ * guest, and thus obtain TDX protections, including PAMT tracking.
+ */
+struct page *tdx_alloc_control_page(void)
+{
+	struct page *page;
+
+	page = alloc_page(GFP_KERNEL_ACCOUNT);
+	if (!page)
+		return NULL;
+
+	if (tdx_pamt_get(page_to_pfn(page))) {
+		__free_page(page);
+		return NULL;
+	}
+
+	return page;
+}
+EXPORT_SYMBOL_FOR_KVM(tdx_alloc_control_page);
+
+/*
+ * Free a page that was gifted to the TDX-Module for use as a control
+ * page. After this, the page is no longer protected by TDX.
+ */
+void tdx_free_control_page(struct page *page)
+{
+	if (!page)
+		return;
+
+	tdx_pamt_put(page_to_pfn(page));
+	__free_page(page);
+}
+EXPORT_SYMBOL_FOR_KVM(tdx_free_control_page);
+
 void tdx_sys_disable(void)
 {
 	struct tdx_module_args args = {};
diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
index bdfd0e1e337ac..a886c54decaad 100644
--- a/arch/x86/virt/vmx/tdx/tdx.h
+++ b/arch/x86/virt/vmx/tdx/tdx.h
@@ -48,6 +48,8 @@
 #define TDH_SYS_CONFIG			45
 #define TDH_SYS_SHUTDOWN		52
 #define TDH_SYS_UPDATE			53
+#define TDH_PHYMEM_PAMT_ADD		58
+#define TDH_PHYMEM_PAMT_REMOVE		59
 #define TDH_SYS_DISABLE			69
 
 /*
-- 
2.54.0

Re: [PATCH v9 03/11] x86/virt/tdx: Add tdx_alloc/free_control_page() helpers

[Dave Hansen]

> +/*
> + * Calculate the arg needed for operating on the DPAMT backing for
> + * a given 4KB page.
> + */
> +static u64 pamt_2mb_arg(kvm_pfn_t pfn)
> +{
> +	/* Arg value will specify a 2MB region of physical address space. */
> +	unsigned long hpa_2mb = ALIGN_DOWN(pfn << PAGE_SHIFT, PMD_SIZE);
> +
> +	return hpa_2mb | TDX_PS_2M;
> +}

/* Helper for building dynamic PAMT seamcall() arguments. */
static u64 pamt_2mb_arg(kvm_pfn_t pfn)
{
	/* Find the 2MB-wide DPAMT region for 'pfn':  */
	unsigned long hpa_2mb = ALIGN_DOWN(pfn << PAGE_SHIFT, PMD_SIZE);

	/* Talk about why the flag is needed... */
	return hpa_2mb | TDX_PS_2M;
}


> +/* Add PAMT backing for the 2MB region surrounding the given pfn. */
> +static u64 tdh_phymem_pamt_add(kvm_pfn_t pfn, struct page **pamt_pages)
> +{
> +	struct tdx_module_args args = {
> +		.rcx = pamt_2mb_arg(pfn),
> +		.rdx = page_to_phys(pamt_pages[0]),
> +		.r8 = page_to_phys(pamt_pages[1]),

Super nit:          ^ should be vertically aligned

> +	};
> +
> +	return seamcall(TDH_PHYMEM_PAMT_ADD, &args);
> +}
> +
> +/* Remove PAMT backing for the 2MB region surrounding the given pfn. */
> +static u64 tdh_phymem_pamt_remove(kvm_pfn_t pfn, struct page **pamt_pages)
> +{
> +	struct tdx_module_args args = {
> +		.rcx = pamt_2mb_arg(pfn),
> +	};
> +	u64 ret;
> +
> +	ret = seamcall_ret(TDH_PHYMEM_PAMT_REMOVE, &args);
> +	if (ret)
> +		return ret;
> +
> +	/* Copy PAMT pages out of the struct per the TDX ABI */
> +	pamt_pages[0] = phys_to_page(args.rdx);
> +	pamt_pages[1] = phys_to_page(args.r8);
> +
> +	return 0;
> +}
> +
> +/* Allocate PAMT memory for the given page */
> +static int tdx_pamt_get(kvm_pfn_t pfn)
> +{
> +	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT];
> +	u64 tdx_status;
> +	int ret;
> +
> +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> +		return 0;
> +
> +	ret = alloc_pamt_array(pamt_pages);
> +	if (ret)
> +		return ret;
> +
> +	tdx_status = tdh_phymem_pamt_add(pfn, pamt_pages);
> +	if (tdx_status != TDX_SUCCESS) {
> +		ret = -EIO;
> +		goto out_free;
> +	}
> +
> +	return 0;
> +
> +out_free:
> +	free_pamt_array(pamt_pages);
> +
> +	return ret;
> +}
> +
> +/* Free PAMT memory for the given page */
> +static void tdx_pamt_put(kvm_pfn_t pfn)
> +{
> +	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT] = {};
> +	u64 tdx_status;
> +
> +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> +		return;
> +
> +	tdx_status = tdh_phymem_pamt_remove(pfn, pamt_pages);
> +
> +	/*
> +	 * Don't free pamt_pages as it could hold garbage when
> +	 * tdh_phymem_pamt_remove() fails.  Don't panic/BUG_ON(), as
> +	 * there is no risk of data corruption, but do yell loudly as
> +	 * failure indicates a kernel bug, memory is being leaked, and
> +	 * the dangling PAMT entry may cause future operations to fail.
> +	 */
> +	if (WARN_ON_ONCE(tdx_status != TDX_SUCCESS))
> +		return;0

Whose fault is the bug here? Probably the TDX module?

> +	free_pamt_array(pamt_pages);
> +}
> +
> +/*
> + * Return a page that can be gifted to the TDX-Module for use as a "control"
> + * page, i.e. pages that are used for control structures for a given TDX
> + * guest, and thus obtain TDX protections, including PAMT tracking.
> + */
> +struct page *tdx_alloc_control_page(void)
> +{
> +	struct page *page;
> +
> +	page = alloc_page(GFP_KERNEL_ACCOUNT);
> +	if (!page)
> +		return NULL;
> +
> +	if (tdx_pamt_get(page_to_pfn(page))) {
> +		__free_page(page);
> +		return NULL;
> +	}

Here's where I start to get lost.

What are the rules around tdx_pamt_get()? tdx_alloc_control_page() is
obviously for one 4k page. But tdx_pamt_get() does an allocation for 2MB
regions. So it has to be able to fail to actually add PAMT in some
cases, gracefully.

Right?

How does that happen?

I know the changelog tried to hand wave this away a bit. But the
comments in here need to say something about tdx_pamt_get() being
unusable on its own. The normal convention would be to __ it, too.

> +	return page;
> +}
> +EXPORT_SYMBOL_FOR_KVM(tdx_alloc_control_page);
> +
> +/*
> + * Free a page that was gifted to the TDX-Module for use as a control

Nit "TDX Module" for consistency.

> + * page. After this, the page is no longer protected by TDX.
> + */
> +void tdx_free_control_page(struct page *page)
> +{
> +	if (!page)
> +		return;
> +
> +	tdx_pamt_put(page_to_pfn(page));
> +	__free_page(page);
> +}
> +EXPORT_SYMBOL_FOR_KVM(tdx_free_control_page);
> +
>  void tdx_sys_disable(void)
>  {
>  	struct tdx_module_args args = {};
> diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
> index bdfd0e1e337ac..a886c54decaad 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.h
> +++ b/arch/x86/virt/vmx/tdx/tdx.h
> @@ -48,6 +48,8 @@
>  #define TDH_SYS_CONFIG			45
>  #define TDH_SYS_SHUTDOWN		52
>  #define TDH_SYS_UPDATE			53
> +#define TDH_PHYMEM_PAMT_ADD		58
> +#define TDH_PHYMEM_PAMT_REMOVE		59
>  #define TDH_SYS_DISABLE			69
>  
>  /*

Re: [PATCH v9 03/11] x86/virt/tdx: Add tdx_alloc/free_control_page() helpers

[Dave Hansen]

On 8/6/26 10:17, Dave Hansen wrote:
> I know the changelog tried to hand wave this away a bit. But the
> comments in here need to say something about tdx_pamt_get() being
> unusable on its own. The normal convention would be to __ it, too.

BTW, here's what I'm looking for. Something like:

/*
 * Allocate Dynamic PAMT memory for the given page.
 *
 * Only call this when the pfn is known not to already have Dynamic
 * PAMT pages in the TDX module for it.
 */
static int __tdx_pamt_get(kvm_pfn_t pfn)
{
	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT];
	u64 tdx_status;
	...


See? Let's talk about the rules around when this can and should be called.

Re: [PATCH v9 03/11] x86/virt/tdx: Add tdx_alloc/free_control_page() helpers

[Edgecombe, Rick P]

On Thu, 2026-08-06 at 10:17 -0700, Dave Hansen wrote:
> > +/*
> > + * Calculate the arg needed for operating on the DPAMT backing for
> > + * a given 4KB page.
> > + */
> > +static u64 pamt_2mb_arg(kvm_pfn_t pfn)
> > +{
> > +	/* Arg value will specify a 2MB region of physical address space.
> > */
> > +	unsigned long hpa_2mb = ALIGN_DOWN(pfn << PAGE_SHIFT, PMD_SIZE);
> > +
> > +	return hpa_2mb | TDX_PS_2M;
> > +}
> 
> /* Helper for building dynamic PAMT seamcall() arguments. */

Hmm ok. I'm seeing a difference in answering "what is this thing" vs an
imperative command to the function to exist. And I'll change the comment to say
DPAMT.

> static u64 pamt_2mb_arg(kvm_pfn_t pfn)
> {
> 	/* Find the 2MB-wide DPAMT region for 'pfn':  */
> 	unsigned long hpa_2mb = ALIGN_DOWN(pfn << PAGE_SHIFT, PMD_SIZE);
> 
> 	/* Talk about why the flag is needed... */
> 	return hpa_2mb | TDX_PS_2M;
> }
> 
> 
> > +/* Add PAMT backing for the 2MB region surrounding the given pfn. */
> > +static u64 tdh_phymem_pamt_add(kvm_pfn_t pfn, struct page **pamt_pages)
> > +{
> > +	struct tdx_module_args args = {
> > +		.rcx = pamt_2mb_arg(pfn),
> > +		.rdx = page_to_phys(pamt_pages[0]),
> > +		.r8 = page_to_phys(pamt_pages[1]),
> 
> Super nit:          ^ should be vertically aligned

Ah, yes. This is a recurring style comment at this point. Will adapt.

> 
> > +	};
> > +
> > +	return seamcall(TDH_PHYMEM_PAMT_ADD, &args);
> > +}
> > +
> > +/* Remove PAMT backing for the 2MB region surrounding the given pfn. */
> > +static u64 tdh_phymem_pamt_remove(kvm_pfn_t pfn, struct page **pamt_pages)
> > +{
> > +	struct tdx_module_args args = {
> > +		.rcx = pamt_2mb_arg(pfn),
> > +	};
> > +	u64 ret;
> > +
> > +	ret = seamcall_ret(TDH_PHYMEM_PAMT_REMOVE, &args);
> > +	if (ret)
> > +		return ret;
> > +
> > +	/* Copy PAMT pages out of the struct per the TDX ABI */
> > +	pamt_pages[0] = phys_to_page(args.rdx);
> > +	pamt_pages[1] = phys_to_page(args.r8);
> > +
> > +	return 0;
> > +}
> > +
> > +/* Allocate PAMT memory for the given page */
> > +static int tdx_pamt_get(kvm_pfn_t pfn)
> > +{
> > +	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT];
> > +	u64 tdx_status;
> > +	int ret;
> > +
> > +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> > +		return 0;
> > +
> > +	ret = alloc_pamt_array(pamt_pages);
> > +	if (ret)
> > +		return ret;
> > +
> > +	tdx_status = tdh_phymem_pamt_add(pfn, pamt_pages);
> > +	if (tdx_status != TDX_SUCCESS) {
> > +		ret = -EIO;
> > +		goto out_free;
> > +	}
> > +
> > +	return 0;
> > +
> > +out_free:
> > +	free_pamt_array(pamt_pages);
> > +
> > +	return ret;
> > +}
> > +
> > +/* Free PAMT memory for the given page */
> > +static void tdx_pamt_put(kvm_pfn_t pfn)
> > +{
> > +	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT] = {};
> > +	u64 tdx_status;
> > +
> > +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> > +		return;
> > +
> > +	tdx_status = tdh_phymem_pamt_remove(pfn, pamt_pages);
> > +
> > +	/*
> > +	 * Don't free pamt_pages as it could hold garbage when
> > +	 * tdh_phymem_pamt_remove() fails.  Don't panic/BUG_ON(), as
> > +	 * there is no risk of data corruption, but do yell loudly as
> > +	 * failure indicates a kernel bug, memory is being leaked, and
> > +	 * the dangling PAMT entry may cause future operations to fail.
> > +	 */
> > +	if (WARN_ON_ONCE(tdx_status != TDX_SUCCESS))
> > +		return;0
> 
> Whose fault is the bug here? Probably the TDX module?

In the code at the end, it is the TDX module's fault. But in this patch is the
caller. Since we don't have refcounts. More on how to handle this below.

> 
> > +	free_pamt_array(pamt_pages);
> > +}
> > +
> > +/*
> > + * Return a page that can be gifted to the TDX-Module for use as a
> > "control"
> > + * page, i.e. pages that are used for control structures for a given TDX
> > + * guest, and thus obtain TDX protections, including PAMT tracking.
> > + */
> > +struct page *tdx_alloc_control_page(void)
> > +{
> > +	struct page *page;
> > +
> > +	page = alloc_page(GFP_KERNEL_ACCOUNT);
> > +	if (!page)
> > +		return NULL;
> > +
> > +	if (tdx_pamt_get(page_to_pfn(page))) {
> > +		__free_page(page);
> > +		return NULL;
> > +	}
> 
> Here's where I start to get lost.
> 
> What are the rules around tdx_pamt_get()? tdx_alloc_control_page() is
> obviously for one 4k page. But tdx_pamt_get() does an allocation for 2MB
> regions. So it has to be able to fail to actually add PAMT in some
> cases, gracefully.
> 
> Right?
> 
> How does that happen?

This used to be combined with the next patch. I thought it was still a bit big
and split it. In the combined patch it was a fully formed functional
implementation.

But both you and Sashiko raised concerns around what actually is the rules for
this. The problem is that the code is not *really* built incrementally, where
each step is functional. Instead, just the code is split between two patches.
Now it feels a bit lazy in retrospect. So I need to either rearrange this so
that each step is a real function that can be reviewed. Or squash them again.

I'm leaning towards shifting code around between the two patches and leaving
them separate. For this one it can have the SEAMCALLs themselves and the get/put
helpers. I'll take the warnings to the next patch, so a caller could take care
themselves to not call them for pages within the same huge mapping. This is
feasible if the caller is already holding a whole huge page, but want to give it
as 4KB to the TDX module. So a reviewer can make more sense of it.

Then I'll move the tdx_alloc_control_page() and friend to the next patch, as
there is no real way to use those helpers unless you only call them once. Until
the refcounts appear. But if they can only be called once, there is no point for
a helper.

At least that's my idea. I'll see how it fits together.

> 
> I know the changelog tried to hand wave this away a bit. But the
> comments in here need to say something about tdx_pamt_get() being
> unusable on its own. The normal convention would be to __ it, too.
> 
> > +	return page;
> > +}
> > +EXPORT_SYMBOL_FOR_KVM(tdx_alloc_control_page);
> > +
> > +/*
> > + * Free a page that was gifted to the TDX-Module for use as a control
> 
> Nit "TDX Module" for consistency.

Oh yep.

> 
> > + * page. After this, the page is no longer protected by TDX.
> > + */
> > +void tdx_free_control_page(struct page *page)
> > +{
> > +	if (!page)
> > +		return;
> > +
> > +	tdx_pamt_put(page_to_pfn(page));
> > +	__free_page(page);
> > +}
> > +EXPORT_SYMBOL_FOR_KVM(tdx_free_control_page);
> > +
> >  void tdx_sys_disable(void)
> >  {
> >  	struct tdx_module_args args = {};
> > diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
> > index bdfd0e1e337ac..a886c54decaad 100644
> > --- a/arch/x86/virt/vmx/tdx/tdx.h
> > +++ b/arch/x86/virt/vmx/tdx/tdx.h
> > @@ -48,6 +48,8 @@
> >  #define TDH_SYS_CONFIG			45
> >  #define TDH_SYS_SHUTDOWN		52
> >  #define TDH_SYS_UPDATE			53
> > +#define TDH_PHYMEM_PAMT_ADD		58
> > +#define TDH_PHYMEM_PAMT_REMOVE		59
> >  #define TDH_SYS_DISABLE			69
> >  
> >  /*
> 

Re: [PATCH v9 03/11] x86/virt/tdx: Add tdx_alloc/free_control_page() helpers

[Dave Hansen]

On 8/6/26 15:22, Edgecombe, Rick P wrote:
> I'm leaning towards shifting code around between the two patches and leaving
> them separate.

Separate is fine. Just name thing low-level function better and describe
how it is supposed to be used.

[PATCH v9 04/11] x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

The PAMT memory holds metadata for all possible TDX protected memory. Each
physical address range is covered by PAMT entries at three levels (1GB,
2MB, 4KB). With Dynamic PAMT, the 4KB level of PAMT is allocated on
demand. The kernel supplies the TDX module with page pairs to store the
4KB level entries, which cover 2MB of host physical memory. The kernel must
provide this page pair before using pages from the range for TDX. If this
is not done, SEAMCALLs that give the pages to be protected by the TDX
module will fail.

Allocate reference counters for every 2MB range to track TDX memory usage.
This can be used to handle concurrent get/put callers, in order to
accurately determine when the dynamic 4KB level of Dynamic PAMT needs to
be allocated and when it can be freed.

This allocation will currently consume 2MB for every 1TB of address
space from 0 to max_pfn. The allocation size will depend on how the RAM is
physically laid out. In a worst case scenario where the entire 52 bit
address space is covered this would be 8GB. Then the Dynamic PAMT refcount
allocations could hypothetically cause the savings from Dynamic PAMT to go
negative on exotic platforms with sparse, small amounts of memory.

Future changes could reduce this refcount overhead to be only allocating
refcounts for physical ranges that contain memory that TDX can use.
However, this is left for future work.

AI was used under supervision to collect/apply feedback, review code and
workshop logs.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
---
v7:
 - Annote functions __init (Chao)
 - Log tweaks (Yan)
 - Stanardize on memory units in the text (Sohil)
 - Delete unneeded comment (Sohil)
 - Use vzalloc() (Sohil)
 - Drop Assisted-by tag and cover AI use in log (Dave)

v6:
 - Remove confusing reference to allocating PAMT memory in
   pamt_refcounts comment. (Yan)
 - Rename "metadata" function names that really deal with refcounts, as
   metadata already has a different meaning in TDX.
 - Move tdx_find_pamt_refcount() to this patch to aid in reviewability
---
 arch/x86/virt/vmx/tdx/tdx.c | 53 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 52 insertions(+), 1 deletion(-)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 4d3427b8a5b71..b0ebd74a69fe2 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -30,6 +30,7 @@
 #include <linux/suspend.h>
 #include <linux/syscore_ops.h>
 #include <linux/idr.h>
+#include <linux/vmalloc.h>
 #include <asm/page.h>
 #include <asm/special_insns.h>
 #include <asm/msr-index.h>
@@ -63,6 +64,14 @@ static DEFINE_PER_CPU(bool, tdx_lp_initialized);
 
 static struct tdmr_info_list tdx_tdmr_list;
 
+/*
+ * On a machine with Dynamic PAMT, the kernel maintains a reference counter
+ * for every 2MB range. The counter indicates how many users there are for
+ * the PAMT memory of the 2MB range. The kernel allocates PAMT refcounts at
+ * initialization.
+ */
+static atomic_t *pamt_refcounts;
+
 /* All TDX-usable memory regions.  Protected by mem_hotplug_lock. */
 static LIST_HEAD(tdx_memlist);
 
@@ -252,6 +261,42 @@ static struct syscore tdx_syscore = {
 	.ops = &tdx_syscore_ops,
 };
 
+/*
+ * Allocate PAMT reference counters for all physical memory.
+ *
+ * It consumes 2MB for every 1TB of physical memory.
+ */
+static __init int init_pamt_refcounts(void)
+{
+	size_t size = DIV_ROUND_UP(max_pfn, PTRS_PER_PTE) * sizeof(*pamt_refcounts);
+
+	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
+		return 0;
+
+	pamt_refcounts = vzalloc(size);
+	if (!pamt_refcounts)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static __init void free_pamt_refcounts(void)
+{
+	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
+		return;
+
+	vfree(pamt_refcounts);
+	pamt_refcounts = NULL;
+}
+
+static atomic_t * __maybe_unused tdx_find_pamt_refcount(unsigned long pfn)
+{
+	/* Find which PMD a PFN is in. */
+	unsigned long index = pfn >> (PMD_SHIFT - PAGE_SHIFT);
+
+	return &pamt_refcounts[index];
+}
+
 /*
  * Add a memory region as a TDX memory block.  The caller must make sure
  * all memory regions are added in address ascending order and don't
@@ -1152,10 +1197,14 @@ static __init int init_tdx_module(void)
 	 */
 	get_online_mems();
 
-	ret = build_tdx_memlist(&tdx_memlist);
+	ret = init_pamt_refcounts();
 	if (ret)
 		goto out_put_tdxmem;
 
+	ret = build_tdx_memlist(&tdx_memlist);
+	if (ret)
+		goto err_free_pamt_refcounts;
+
 	/* Allocate enough space for constructing TDMRs */
 	ret = alloc_tdmr_list(&tdx_tdmr_list, &tdx_sysinfo.tdmr);
 	if (ret)
@@ -1205,6 +1254,8 @@ static __init int init_tdx_module(void)
 	free_tdmr_list(&tdx_tdmr_list);
 err_free_tdxmem:
 	free_tdx_memlist(&tdx_memlist);
+err_free_pamt_refcounts:
+	free_pamt_refcounts();
 	goto out_put_tdxmem;
 }
 
-- 
2.54.0

Re: [PATCH v9 04/11] x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] vzalloc() lacks __GFP_RETRY_MAYFAIL, causing it to invoke the OOM killer and panic the kernel during boot on systems with sparse memory layouts.
--

--- Patch [4]: [PATCH v9 04/11] x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory ---
commit b63d4342b3d95af2bbe50ca5c66ac1ecbf04dedc
Author: Kiryl Shutsemau <kas@kernel.org>

x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory

This commit allocates reference counters for every 2MB memory range to track
TDX memory usage. The allocation size is based on max_pfn and can be very 
large on platforms with sparse memory layouts. If allocation fails, it is
intended to return -ENOMEM and fail TDX initialization.

> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 4d3427b8a5b71..b0ebd74a69fe2 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
[ ... ]
> @@ -252,6 +261,42 @@ static struct syscore tdx_syscore = {
>  	.ops = &tdx_syscore_ops,
>  };
>  
> +/*
> + * Allocate PAMT reference counters for all physical memory.
> + *
> + * It consumes 2MB for every 1TB of physical memory.
> + */
> +static __init int init_pamt_refcounts(void)
> +{
> +	size_t size = DIV_ROUND_UP(max_pfn, PTRS_PER_PTE) * sizeof(*pamt_refcounts);
> +
> +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> +		return 0;
> +
> +	pamt_refcounts = vzalloc(size);
> +	if (!pamt_refcounts)
> +		return -ENOMEM;

[Severity: High]
Could this vzalloc() call trigger a kernel panic on systems with sparse
memory layouts instead of gracefully disabling TDX?

The allocation size based on max_pfn can be extremely large (up to 8GB).
Because vzalloc() does not include __GFP_RETRY_MAYFAIL, an allocation
failure will invoke the page allocator slow path and the OOM killer. Since
this runs during early boot before any killable user processes exist, the
OOM killer will unconditionally call panic() instead of returning NULL.

Would it be safer to use __vmalloc() with GFP_KERNEL | __GFP_ZERO |
__GFP_RETRY_MAYFAIL | __GFP_NOWARN to ensure the allocator returns NULL
when memory is exhausted?

> +
> +	return 0;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260806020850.1221381-1-rick.p.edgecombe@intel.com?part=4

Re: [PATCH v9 04/11] x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory

[Edgecombe, Rick P]

CC everyone on this Sashiko comment, because I think its not 100% clear cut how
to handle it.

On Thu, 2026-08-06 at 02:21 +0000, sashiko-bot@kernel.org wrote:
> [Severity: High]
> Could this vzalloc() call trigger a kernel panic on systems with sparse
> memory layouts instead of gracefully disabling TDX?
> 
> The allocation size based on max_pfn can be extremely large (up to 8GB).
> Because vzalloc() does not include __GFP_RETRY_MAYFAIL, an allocation
> failure will invoke the page allocator slow path and the OOM killer. Since
> this runs during early boot before any killable user processes exist, the
> OOM killer will unconditionally call panic() instead of returning NULL.
> 
> Would it be safer to use __vmalloc() with GFP_KERNEL | __GFP_ZERO |
> __GFP_RETRY_MAYFAIL | __GFP_NOWARN to ensure the allocator returns NULL
> when memory is exhausted?

I think this is a bit overblown. If there is a lot of memory such that this
allocation is large, we can expect it be less likely to fail.

So I'm not sure it needs special handling. The tradeoff is more non-standard
code to read and consider, weighed against what I think is probably a low cost
of hitting the problem. So I think I'll leave it. Especially since DPAMT is an
opt-in, there is an escape hatch for now. So if the reasoning is very wrong,
impact is not horrible.

Re: [PATCH v9 04/11] x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory

[Dave Hansen]

On 8/6/26 15:02, Edgecombe, Rick P wrote:
> On Thu, 2026-08-06 at 02:21 +0000, sashiko-bot@kernel.org wrote:
>> [Severity: High]
>> Could this vzalloc() call trigger a kernel panic on systems with sparse
>> memory layouts instead of gracefully disabling TDX?
...
> I think this is a bit overblown. If there is a lot of memory such that this
> allocation is large, we can expect it be less likely to fail.

Yes, it could trigger a kernel panic. And, yes, sashiko's concern is
overblown.

This code only runs on the host side, so there's no exotic VM craziness.
In reality, today, there are not many TDX systems. The ones that do
exist don't have exotic sparse memory layouts either. TDX also doesn't
yet support any of the emerging memory technologies like CXL.mem that
might lead to bare-metal weirdo layouts.

But even in a future with a pathologically evil memory layout, is anyone
going to miss 8GB of memory on a server? Probably not.

So, yeah, I just don't see the point of going crazy over this. If
someone hits this in 10 years, we'll fix it then. Oooh, maybe I'll be
retired in 10 years. One can dream.

Re: [PATCH v9 04/11] x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory

[Dave Hansen]

On 8/5/26 19:08, Rick Edgecombe wrote:
> + * Allocate PAMT reference counters for all physical memory.

Nit for all of these: This has *ZERO* to do with the "PAMT", which is an
ancient TDX concept. This new gunk is 100% for *dynamic* PAMT.

Could we please be clear about that universally?

I think just saying "DPAMT" in most cases this says "PAMT" will help.

Re: [PATCH v9 04/11] x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory

[Edgecombe, Rick P]

On Thu, 2026-08-06 at 13:56 -0700, Dave Hansen wrote:
> On 8/5/26 19:08, Rick Edgecombe wrote:
> > + * Allocate PAMT reference counters for all physical memory.
> 
> Nit for all of these: This has *ZERO* to do with the "PAMT", which is an
> ancient TDX concept. This new gunk is 100% for *dynamic* PAMT.
> 
> Could we please be clear about that universally?
> 
> I think just saying "DPAMT" in most cases this says "PAMT" will help.

Ok, I'll standardize on DPAMT. I was previously trying to standardize on
"Dynamic PAMT" spelled out and missed this one because it was PAMT.

So I'll do the first reference in the logs as "Dynamic PAMT (DPAMT)" and in the
code just DPAMT.

[PATCH v9 05/11] x86/virt/tdx: Handle multiple callers in tdx_pamt_get/put()

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

tdx_pamt_get()/tdx_pamt_put() unconditionally add or remove Dynamic PAMT
backing for the 2MB region covering the passed page. However, multiple
callers can add or remove 4KB pages that fall within the same 2MB region
and in that scenario only a single PAMT entry is required.

Make the helpers handle only adding/removing Dynamic PAMT backing when
required, by refcounting each 2MB range. Gate the actual Dynamic PAMT add
and remove on refcount transitions (0->1 and 1->0). Serialize the refcount
check and SEAMCALL with a global spinlock so the read-decide-act sequence
is atomic. This also avoids TDX module BUSY errors, as the Dynamic PAMT add
and remove SEAMCALLs take internal TDX module locks for the 2MB ranges of
the specified PFN and the PAMT page pair PFNs. So simultaneous attempts on
the same 2MB ranges of the PFNs would otherwise encounter an error, which
would not be handleable in the put case.

The lock is global and heavyweight. Use simple conditional logic to keep
correctness obvious. This will be optimized in a later change.

The pamt_refcounts[] are atomic_t's. They do not strictly need to be
because all access is protected by pamt_lock. The overhead of an atomic_t
in this situation is minuscule compared to the global lock. Leave the
atomic_t in place to enable future optimization with minimal churn.

AI was used under supervision to collect/apply feedback, split patches,
review code and workshop logs.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
---
v8:
 - Fix PAMT capitalization in comment (Sohil)

v7:
 - Convert scoped_guard() blocks to use normal spin_un/lock() for the
   sake of making next patches diff cleaner
 - Drop __maybe_unused from tdx_find_pamt_refcount() (Binbin)
 - Switch to atomic_inc_not_zero() (Dave)
 - Justify use of atomic_t in log (Sohil)
 - Log/comments (Yan)
 - Drop Assisted-by tag and cover AI use in log (Dave)
---
 arch/x86/virt/vmx/tdx/tdx.c | 48 +++++++++++++++++++++++++++++++++----
 1 file changed, 43 insertions(+), 5 deletions(-)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index b0ebd74a69fe2..0e6063c1405bc 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -289,7 +289,7 @@ static __init void free_pamt_refcounts(void)
 	pamt_refcounts = NULL;
 }
 
-static atomic_t * __maybe_unused tdx_find_pamt_refcount(unsigned long pfn)
+static atomic_t *tdx_find_pamt_refcount(unsigned long pfn)
 {
 	/* Find which PMD a PFN is in. */
 	unsigned long index = pfn >> (PMD_SHIFT - PAGE_SHIFT);
@@ -2120,10 +2120,14 @@ static u64 tdh_phymem_pamt_remove(kvm_pfn_t pfn, struct page **pamt_pages)
 	return 0;
 }
 
-/* Allocate PAMT memory for the given page */
+/* Serializes adding/removing PAMT memory */
+static DEFINE_SPINLOCK(pamt_lock);
+
+/* Bump PAMT refcount for the given pfn and allocate PAMT backing if needed. */
 static int tdx_pamt_get(kvm_pfn_t pfn)
 {
 	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT];
+	atomic_t *pamt_refcount;
 	u64 tdx_status;
 	int ret;
 
@@ -2134,29 +2138,58 @@ static int tdx_pamt_get(kvm_pfn_t pfn)
 	if (ret)
 		return ret;
 
+	pamt_refcount = tdx_find_pamt_refcount(pfn);
+
+	spin_lock(&pamt_lock);
+
+	/*
+	 * If the pamt page is already added (i.e. refcount >= 1),
+	 * then just increment the refcount.
+	 */
+	if (atomic_inc_not_zero(pamt_refcount))
+		goto out_free;
+
+	/* Try to add the PAMT page and take the refcount 0->1. */
 	tdx_status = tdh_phymem_pamt_add(pfn, pamt_pages);
-	if (tdx_status != TDX_SUCCESS) {
+	if (WARN_ON_ONCE(tdx_status != TDX_SUCCESS)) {
 		ret = -EIO;
 		goto out_free;
 	}
 
+	atomic_set(pamt_refcount, 1);
+	spin_unlock(&pamt_lock);
 	return 0;
 
 out_free:
+	spin_unlock(&pamt_lock);
 	free_pamt_array(pamt_pages);
 
 	return ret;
 }
 
-/* Free PAMT memory for the given page */
+/* Drop PAMT refcount for the given pfn and free PAMT backing if needed. */
 static void tdx_pamt_put(kvm_pfn_t pfn)
 {
 	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT] = {};
+	atomic_t *pamt_refcount;
 	u64 tdx_status;
 
 	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
 		return;
 
+	pamt_refcount = tdx_find_pamt_refcount(pfn);
+
+	spin_lock(&pamt_lock);
+	/*
+	 * If there is more than 1 reference on the pamt page, don't
+	 * remove it yet. Just decrement the refcount.
+	 */
+	if (atomic_read(pamt_refcount) > 1) {
+		atomic_dec(pamt_refcount);
+		goto out_unlock;
+	}
+
+	/* Try to remove the pamt page and take the refcount 1->0. */
 	tdx_status = tdh_phymem_pamt_remove(pfn, pamt_pages);
 
 	/*
@@ -2167,9 +2200,14 @@ static void tdx_pamt_put(kvm_pfn_t pfn)
 	 * the dangling PAMT entry may cause future operations to fail.
 	 */
 	if (WARN_ON_ONCE(tdx_status != TDX_SUCCESS))
-		return;
+		goto out_unlock;
 
+	atomic_set(pamt_refcount, 0);
+	spin_unlock(&pamt_lock);
 	free_pamt_array(pamt_pages);
+	return;
+out_unlock:
+	spin_unlock(&pamt_lock);
 }
 
 /*
-- 
2.54.0

Re: [PATCH v9 05/11] x86/virt/tdx: Handle multiple callers in tdx_pamt_get/put()

[Dave Hansen]

On 8/5/26 19:08, Rick Edgecombe wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> 
> tdx_pamt_get()/tdx_pamt_put() unconditionally add or remove Dynamic PAMT
> backing for the 2MB region covering the passed page. However, multiple
> callers can add or remove 4KB pages that fall within the same 2MB region
> and in that scenario only a single PAMT entry is required.

Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

[PATCH v9 06/11] KVM: TDX: Allocate PAMT memory for TD and vCPU control structures

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Use control page helpers for allocating and freeing TD control structures,
such that these operations can work for Dynamic PAMT.

The TDX module tracks some state for each page of physical memory that it
might use. It calls this state the PAMT. It includes separate state for
each page size a physical page could be utilized at within the TDX module
(1GB, 2MB, 4KB). In Dynamic PAMT, only the 4KB page size state is
allocated dynamically. So the kernel must ensure PAMT backing is installed
for any 4KB page being gifted to the TDX module, and must tear down the
backing when all associated gifted pages are reclaimed.

TD scoped control pages (TDR, TDCS) and vCPU scoped control pages (TDVPR,
TDCX) are all handed to the TDX module at 4KB page size and are therefore
subject to this requirement. Replace the raw alloc_page()/__free_page()
calls for these pages with tdx_alloc/free_control_page().

Switching between special Dynamic PAMT operations or normal page
alloc/free operations is handled internally in
tdx_alloc/free_control_page(). So don't check for Dynamic PAMT around these
calls. Just call them unconditionally. Similarly, drop the NULL checks
before freeing, as tdx_free_control_page() handles NULL internally.

No functional change intended when Dynamic PAMT is not in use.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
[sean: handle alloc+free+reclaim in one patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>
[rick: enhance log, reviewing, rebase, with help from AI tooling]
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Sean Christopherson <seanjc@google.com>
---
v7:
 - Fixup tags (Sean)
 - Missing word in log (Binbin)
 - Log smoothness (Yan)
---
 arch/x86/kvm/vmx/tdx.c | 35 ++++++++++++++---------------------
 1 file changed, 14 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 545b03d9d10b8..957078e8656ef 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -362,7 +362,7 @@ static void tdx_reclaim_control_page(struct page *ctrl_page)
 	if (tdx_reclaim_page(ctrl_page))
 		return;
 
-	__free_page(ctrl_page);
+	tdx_free_control_page(ctrl_page);
 }
 
 struct tdx_flush_vp_arg {
@@ -589,7 +589,7 @@ static void tdx_reclaim_td_control_pages(struct kvm *kvm)
 
 	tdx_quirk_reset_paddr(page_to_phys(kvm_tdx->td.tdr_page), PAGE_SIZE);
 
-	__free_page(kvm_tdx->td.tdr_page);
+	tdx_free_control_page(kvm_tdx->td.tdr_page);
 	kvm_tdx->td.tdr_page = NULL;
 }
 
@@ -2459,7 +2459,7 @@ static int __tdx_td_init(struct kvm *kvm, struct td_params *td_params,
 
 	ret = -ENOMEM;
 
-	tdr_page = alloc_page(GFP_KERNEL_ACCOUNT);
+	tdr_page = tdx_alloc_control_page();
 	if (!tdr_page)
 		goto free_hkid;
 
@@ -2472,7 +2472,7 @@ static int __tdx_td_init(struct kvm *kvm, struct td_params *td_params,
 		goto free_tdr;
 
 	for (i = 0; i < kvm_tdx->td.tdcs_nr_pages; i++) {
-		tdcs_pages[i] = alloc_page(GFP_KERNEL_ACCOUNT);
+		tdcs_pages[i] = tdx_alloc_control_page();
 		if (!tdcs_pages[i])
 			goto free_tdcs;
 	}
@@ -2590,10 +2590,8 @@ static int __tdx_td_init(struct kvm *kvm, struct td_params *td_params,
 teardown:
 	/* Only free pages not yet added, so start at 'i' */
 	for (; i < kvm_tdx->td.tdcs_nr_pages; i++) {
-		if (tdcs_pages[i]) {
-			__free_page(tdcs_pages[i]);
-			tdcs_pages[i] = NULL;
-		}
+		tdx_free_control_page(tdcs_pages[i]);
+		tdcs_pages[i] = NULL;
 	}
 	if (!kvm_tdx->td.tdcs_pages)
 		kfree(tdcs_pages);
@@ -2608,16 +2606,13 @@ static int __tdx_td_init(struct kvm *kvm, struct td_params *td_params,
 	free_cpumask_var(packages);
 
 free_tdcs:
-	for (i = 0; i < kvm_tdx->td.tdcs_nr_pages; i++) {
-		if (tdcs_pages[i])
-			__free_page(tdcs_pages[i]);
-	}
+	for (i = 0; i < kvm_tdx->td.tdcs_nr_pages; i++)
+		tdx_free_control_page(tdcs_pages[i]);
 	kfree(tdcs_pages);
 	kvm_tdx->td.tdcs_pages = NULL;
 
 free_tdr:
-	if (tdr_page)
-		__free_page(tdr_page);
+	tdx_free_control_page(tdr_page);
 	kvm_tdx->td.tdr_page = NULL;
 
 free_hkid:
@@ -2951,7 +2946,7 @@ static int tdx_td_vcpu_init(struct kvm_vcpu *vcpu, u64 vcpu_rcx)
 	int ret, i;
 	u64 err;
 
-	page = alloc_page(GFP_KERNEL_ACCOUNT);
+	page = tdx_alloc_control_page();
 	if (!page)
 		return -ENOMEM;
 	tdx->vp.tdvpr_page = page;
@@ -2971,7 +2966,7 @@ static int tdx_td_vcpu_init(struct kvm_vcpu *vcpu, u64 vcpu_rcx)
 	}
 
 	for (i = 0; i < kvm_tdx->td.tdcx_nr_pages; i++) {
-		page = alloc_page(GFP_KERNEL_ACCOUNT);
+		page = tdx_alloc_control_page();
 		if (!page) {
 			ret = -ENOMEM;
 			goto free_tdcx;
@@ -2993,7 +2988,7 @@ static int tdx_td_vcpu_init(struct kvm_vcpu *vcpu, u64 vcpu_rcx)
 			 * method, but the rest are freed here.
 			 */
 			for (; i < kvm_tdx->td.tdcx_nr_pages; i++) {
-				__free_page(tdx->vp.tdcx_pages[i]);
+				tdx_free_control_page(tdx->vp.tdcx_pages[i]);
 				tdx->vp.tdcx_pages[i] = NULL;
 			}
 			return -EIO;
@@ -3021,16 +3016,14 @@ static int tdx_td_vcpu_init(struct kvm_vcpu *vcpu, u64 vcpu_rcx)
 
 free_tdcx:
 	for (i = 0; i < kvm_tdx->td.tdcx_nr_pages; i++) {
-		if (tdx->vp.tdcx_pages[i])
-			__free_page(tdx->vp.tdcx_pages[i]);
+		tdx_free_control_page(tdx->vp.tdcx_pages[i]);
 		tdx->vp.tdcx_pages[i] = NULL;
 	}
 	kfree(tdx->vp.tdcx_pages);
 	tdx->vp.tdcx_pages = NULL;
 
 free_tdvpr:
-	if (tdx->vp.tdvpr_page)
-		__free_page(tdx->vp.tdvpr_page);
+	tdx_free_control_page(tdx->vp.tdvpr_page);
 	tdx->vp.tdvpr_page = NULL;
 	tdx->vp.tdvpr_pa = 0;
 
-- 
2.54.0

Re: [PATCH v9 06/11] KVM: TDX: Allocate PAMT memory for TD and vCPU control structures

[Dave Hansen]

On 8/5/26 19:08, Rick Edgecombe wrote:
> Use control page helpers for allocating and freeing TD control structures,
> such that these operations can work for Dynamic PAMT.

Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

[PATCH v9 07/11] x86/tdx: Add APIs to support Dynamic PAMT ops from KVM's fault path

[Rick Edgecombe]

When handling an EPT violation, KVM holds a spinlock while manipulating
the EPT. Before entering the spinlock it doesn't know how many EPT page
tables will need to be installed or whether a huge page will be used. For
this reason it allocates a worst case number of page tables that it might
need as part of servicing the EPT violation.

Under Dynamic PAMT these pre-allocated pages will potentially need to have
Dynamic PAMT backing pages installed for them. KVM already has helpers to
manage topping up page caches before taking the MMU lock, but they cannot
be passed from KVM to arch/x86 code.

The problem of how and when to install the Dynamic PAMT backing pages for
the pages given to the TDX module during the fault path has had a lot of
design attempts.
 - Extracting KVM's MMU caches requires too much inlined code added to
   headers.
 - A few varieties of installing Dynamic PAMT backing when allocating the
   S-EPT page tables. (see links)
 - Using mempool_t to transfer the pages between KVM and arch/x86 doesn't
   work because the component is designed more around maintaining a pool
   of pages, rather than topping up a continually drained cache.

So don't do these as they all had various problems. Instead just create a
small simple data structure to use for handing a pre-allocated list of
pages between KVM and arch/x86 code. Model this on KVM's existing MMU
memory caches.

Add a tdx_pamt_cache arg to tdx_pamt_get() so it can draw pages from a
cache when needed. Not all Dynamic PAMT page installations will happen
under spinlock, for example TD and vCPU scoped control pages. So have
tdx_pamt_get() maintain the existing behavior of allocating from the page
allocator when NULL is passed for the struct tdx_pamt_cache arg. This
prevents excess allocations for cases where it can be avoided.

Export the new helpers for KVM.

AI was used under supervision to review code and workshop logs.

Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/kvm/aXENNKjAKTM9UJNH@google.com/
Link: https://lore.kernel.org/kvm/20260129011517.3545883-20-seanjc@google.com/
Link: https://lore.kernel.org/kvm/aYW5CbUvZrLogsWF@yzhao56-desk.sh.intel.com/
---
v7:
 - Log/comment tweaks (Yan)
 - Drop Assisted-by tag and cover AI use in log (Dave)

v6:
 - Filled out log from Sean's series
---
 arch/x86/include/asm/tdx.h  | 17 ++++++++++
 arch/x86/virt/vmx/tdx/tdx.c | 65 +++++++++++++++++++++++++++++++++----
 2 files changed, 76 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 86bf37b15c705..9cbd250bbd39b 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -120,6 +120,23 @@ static inline bool tdx_supports_runtime_update(const struct tdx_sys_info *sysinf
 
 bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo);
 
+/* Simple structure for pre-allocating Dynamic PAMT pages outside of spinlocks. */
+struct tdx_pamt_cache {
+	struct list_head page_list;
+	int cnt;
+};
+
+static inline void tdx_init_pamt_cache(struct tdx_pamt_cache *cache)
+{
+	INIT_LIST_HEAD(&cache->page_list);
+	cache->cnt = 0;
+}
+
+void tdx_free_pamt_cache(struct tdx_pamt_cache *cache);
+int tdx_topup_pamt_cache(struct tdx_pamt_cache *cache, unsigned long npages);
+int tdx_pamt_get(kvm_pfn_t pfn, struct tdx_pamt_cache *cache);
+void tdx_pamt_put(kvm_pfn_t pfn);
+
 int tdx_guest_keyid_alloc(void);
 u32 tdx_get_nr_guest_keyids(void);
 void tdx_guest_keyid_free(unsigned int keyid);
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 0e6063c1405bc..4d90d1a555135 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -2045,12 +2045,33 @@ bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo)
 	return false;
 }
 
-static int alloc_pamt_array(struct page **pamt_pages)
+static struct page *tdx_alloc_page_pamt_cache(struct tdx_pamt_cache *cache)
+{
+	struct page *page;
+
+	page = list_first_entry_or_null(&cache->page_list, struct page, lru);
+	if (page) {
+		list_del(&page->lru);
+		cache->cnt--;
+	}
+
+	return page;
+}
+
+static struct page *alloc_dpamt_page(struct tdx_pamt_cache *cache)
+{
+	if (cache)
+		return tdx_alloc_page_pamt_cache(cache);
+
+	return alloc_page(GFP_KERNEL_ACCOUNT);
+}
+
+static int alloc_pamt_array(struct page **pamt_pages, struct tdx_pamt_cache *cache)
 {
 	int i, j;
 
 	for (i = 0; i < TDX_DPAMT_ENTRY_PAGE_CNT; i++) {
-		pamt_pages[i] = alloc_page(GFP_KERNEL_ACCOUNT);
+		pamt_pages[i] = alloc_dpamt_page(cache);
 		if (!pamt_pages[i])
 			goto err;
 	}
@@ -2124,7 +2145,7 @@ static u64 tdh_phymem_pamt_remove(kvm_pfn_t pfn, struct page **pamt_pages)
 static DEFINE_SPINLOCK(pamt_lock);
 
 /* Bump PAMT refcount for the given pfn and allocate PAMT backing if needed. */
-static int tdx_pamt_get(kvm_pfn_t pfn)
+int tdx_pamt_get(kvm_pfn_t pfn, struct tdx_pamt_cache *cache)
 {
 	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT];
 	atomic_t *pamt_refcount;
@@ -2134,7 +2155,7 @@ static int tdx_pamt_get(kvm_pfn_t pfn)
 	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
 		return 0;
 
-	ret = alloc_pamt_array(pamt_pages);
+	ret = alloc_pamt_array(pamt_pages, cache);
 	if (ret)
 		return ret;
 
@@ -2166,9 +2187,10 @@ static int tdx_pamt_get(kvm_pfn_t pfn)
 
 	return ret;
 }
+EXPORT_SYMBOL_FOR_KVM(tdx_pamt_get);
 
 /* Drop PAMT refcount for the given pfn and free PAMT backing if needed. */
-static void tdx_pamt_put(kvm_pfn_t pfn)
+void tdx_pamt_put(kvm_pfn_t pfn)
 {
 	struct page *pamt_pages[TDX_DPAMT_ENTRY_PAGE_CNT] = {};
 	atomic_t *pamt_refcount;
@@ -2209,6 +2231,37 @@ static void tdx_pamt_put(kvm_pfn_t pfn)
 out_unlock:
 	spin_unlock(&pamt_lock);
 }
+EXPORT_SYMBOL_FOR_KVM(tdx_pamt_put);
+
+void tdx_free_pamt_cache(struct tdx_pamt_cache *cache)
+{
+	struct page *page;
+
+	while ((page = tdx_alloc_page_pamt_cache(cache)))
+		__free_page(page);
+}
+EXPORT_SYMBOL_FOR_KVM(tdx_free_pamt_cache);
+
+int tdx_topup_pamt_cache(struct tdx_pamt_cache *cache, unsigned long npages)
+{
+	if (WARN_ON_ONCE(!tdx_supports_dynamic_pamt(&tdx_sysinfo)))
+		return 0;
+
+	npages *= TDX_DPAMT_ENTRY_PAGE_CNT;
+
+	while (cache->cnt < npages) {
+		struct page *page = alloc_page(GFP_KERNEL_ACCOUNT);
+
+		if (!page)
+			return -ENOMEM;
+
+		list_add(&page->lru, &cache->page_list);
+		cache->cnt++;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_FOR_KVM(tdx_topup_pamt_cache);
 
 /*
  * Return a page that can be gifted to the TDX-Module for use as a "control"
@@ -2223,7 +2276,7 @@ struct page *tdx_alloc_control_page(void)
 	if (!page)
 		return NULL;
 
-	if (tdx_pamt_get(page_to_pfn(page))) {
+	if (tdx_pamt_get(page_to_pfn(page), NULL)) {
 		__free_page(page);
 		return NULL;
 	}
-- 
2.54.0

Re: [PATCH v9 07/11] x86/tdx: Add APIs to support Dynamic PAMT ops from KVM's fault path

[Dave Hansen]

On 8/5/26 19:08, Rick Edgecombe wrote:
> So don't do these as they all had various problems. Instead just create a
> small simple data structure to use for handing a pre-allocated list of
> pages between KVM and arch/x86 code. Model this on KVM's existing MMU
> memory caches.

The main thing is that the KVM folks are happy with this, so I'm happy too:

Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

[PATCH v9 08/11] KVM: TDX: Get/put PAMT pages when (un)mapping private memory

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Add Dynamic PAMT support to KVM's S-EPT MMU by "getting" a PAMT page when
adding guest memory (PAGE.ADD or PAGE.AUG), and "putting" the page when
removing guest memory (PAGE.REMOVE).

To access the per-vCPU PAMT caches without plumbing @vcpu throughout the
TDP MMU, begrudgingly use kvm_get_running_vcpu() to get the vCPU, and bug
the VM if KVM attempts to set an S-EPT leaf without an active vCPU.  KVM
only supports creating _new_ mappings in page (pre)fault paths, all of
which require an active vCPU.

The PAMT memory holds metadata for TDX protected memory. With Dynamic
PAMT, PAMT_4K is allocated on demand. The kernel supplies the TDX module
with a few pages that cover 2MB of host physical memory.

Releases are balanced via tdx_pamt_put(): every control-page free goes
through tdx_free_control_page(), and guest data pages are put directly on
the successful tdh_mem_page_remove() path and in the
tdx_mem_page_add/aug() error path.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
[rick: enhance log, reviewing, rebase, with help from AI tooling]
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Sean Christopherson <seanjc@google.com>
---
v8:
 - Remove extra new line around *topup_external_cache() (Sean)
 - Make +1/-1 logic clearer by just coding the easily optimizable math.
   (Nikolay, Sean)
 - Do KVM_BUG_ON() on tdx_pamt_get() return. (Sean)

v7:
 - Don't do to_tdx() before NULL check for readability (Binbin, Sean)
 - Fixup tags (Sean)
 - Export tdx_supports_dynamic_pamt() since it's used in KVM here and no
   longer an static inline.
---
 arch/x86/include/asm/kvm-x86-ops.h |  1 +
 arch/x86/include/asm/kvm_host.h    |  1 +
 arch/x86/kvm/mmu/mmu.c             |  4 ++
 arch/x86/kvm/vmx/tdx.c             | 64 ++++++++++++++++++++++++++----
 arch/x86/kvm/vmx/tdx.h             |  2 +
 arch/x86/virt/vmx/tdx/tdx.c        |  1 +
 6 files changed, 65 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index 83dc5086138b3..588563dfe88d5 100644
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -98,6 +98,7 @@ KVM_X86_OP_OPTIONAL_RET0(tdp_has_smep)
 KVM_X86_OP(load_mmu_pgd)
 KVM_X86_OP_OPTIONAL_RET0(set_external_spte)
 KVM_X86_OP_OPTIONAL(free_external_spt)
+KVM_X86_OP_OPTIONAL_RET0(topup_external_cache)
 KVM_X86_OP(has_wbinvd_exit)
 KVM_X86_OP(get_l2_tsc_offset)
 KVM_X86_OP(get_l2_tsc_multiplier)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 5f6c1ce9673b7..0dbf6564dd0c5 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1922,6 +1922,7 @@ struct kvm_x86_ops {
 	/* Update external page tables for page table about to be freed. */
 	void (*free_external_spt)(struct kvm *kvm, struct kvm_mmu_page *sp);
 
+	int (*topup_external_cache)(struct kvm_vcpu *vcpu, int min_nr_spts);
 
 	bool (*has_wbinvd_exit)(void);
 
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 66e69d2a41b3c..2a628b8304409 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -614,6 +614,10 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
 					       PT64_ROOT_MAX_LEVEL);
 		if (r)
 			return r;
+
+		r = kvm_x86_call(topup_external_cache)(vcpu, PT64_ROOT_MAX_LEVEL);
+		if (r)
+			return r;
 	}
 	r = kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
 				       PT64_ROOT_MAX_LEVEL);
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 957078e8656ef..09573112540e9 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -681,6 +681,8 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu)
 	if (!irqchip_split(vcpu->kvm))
 		return -EINVAL;
 
+	tdx_init_pamt_cache(&tdx->pamt_cache);
+
 	fpstate_set_confidential(&vcpu->arch.guest_fpu);
 	vcpu->arch.apic->guest_apic_protected = true;
 	INIT_LIST_HEAD(&tdx->vt.pi_wakeup_list);
@@ -866,6 +868,8 @@ void tdx_vcpu_free(struct kvm_vcpu *vcpu)
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
 	int i;
 
+	tdx_free_pamt_cache(&tdx->pamt_cache);
+
 	if (vcpu->cpu != -1) {
 		KVM_BUG_ON(tdx->state == VCPU_TD_STATE_INITIALIZED, vcpu->kvm);
 		tdx_flush_vp_on_cpu(vcpu);
@@ -1621,6 +1625,17 @@ void tdx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa, int pgd_level)
 	td_vmcs_write64(to_tdx(vcpu), SHARED_EPT_POINTER, root_hpa);
 }
 
+static int tdx_topup_external_pamt_cache(struct kvm_vcpu *vcpu, int min_nr_spts)
+{
+	/*
+	 * Minus one page to exclude the root SPT, but plus one page for a
+	 * possible 4KB private mapping.
+	 */
+	min_nr_spts += -1 + 1;
+
+	return tdx_topup_pamt_cache(&to_tdx(vcpu)->pamt_cache, min_nr_spts);
+}
+
 static int tdx_mem_page_add(struct kvm *kvm, gfn_t gfn, enum pg_level level,
 			    kvm_pfn_t pfn)
 {
@@ -1679,16 +1694,28 @@ static struct page *tdx_spte_to_sept_pt(struct kvm *kvm, gfn_t gfn,
 static int tdx_sept_map_nonleaf_spte(struct kvm *kvm, gfn_t gfn,
 				     enum pg_level level, u64 new_spte)
 {
+	struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
 	gpa_t gpa = gfn_to_gpa(gfn);
 	u64 err, entry, level_state;
 	struct page *sept_pt;
+	int ret;
+
+	if (KVM_BUG_ON(!vcpu, kvm))
+		return -EIO;
 
 	sept_pt = tdx_spte_to_sept_pt(kvm, gfn, new_spte, level);
 	if (!sept_pt)
 		return -EIO;
 
+	ret = tdx_pamt_get(page_to_pfn(sept_pt), &to_tdx(vcpu)->pamt_cache);
+	if (KVM_BUG_ON(ret, kvm))
+		return ret;
+
 	err = tdh_mem_sept_add(&to_kvm_tdx(kvm)->td, gpa, level, sept_pt,
 			       &entry, &level_state);
+	if (err)
+		tdx_pamt_put(page_to_pfn(sept_pt));
+
 	if (unlikely(tdx_operand_busy(err)))
 		return -EBUSY;
 
@@ -1701,8 +1728,13 @@ static int tdx_sept_map_nonleaf_spte(struct kvm *kvm, gfn_t gfn,
 static int tdx_sept_map_leaf_spte(struct kvm *kvm, gfn_t gfn, enum pg_level level,
 				  u64 new_spte)
 {
+	struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
 	kvm_pfn_t pfn = spte_to_pfn(new_spte);
+	int ret;
+
+	if (KVM_BUG_ON(!vcpu, kvm))
+		return -EIO;
 
 	/* TODO: handle large pages. */
 	if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
@@ -1710,6 +1742,10 @@ static int tdx_sept_map_leaf_spte(struct kvm *kvm, gfn_t gfn, enum pg_level leve
 
 	WARN_ON_ONCE((new_spte & VMX_EPT_RWX_MASK) != VMX_EPT_RWX_MASK);
 
+	ret = tdx_pamt_get(pfn, &to_tdx(vcpu)->pamt_cache);
+	if (KVM_BUG_ON(ret, kvm))
+		return ret;
+
 	/*
 	 * Ensure pre_fault_allowed is read by kvm_arch_vcpu_pre_fault_memory()
 	 * before kvm_tdx->state.  Userspace must not be allowed to pre-fault
@@ -1722,10 +1758,15 @@ static int tdx_sept_map_leaf_spte(struct kvm *kvm, gfn_t gfn, enum pg_level leve
 	 * If the TD isn't finalized/runnable, then userspace is initializing
 	 * the VM image via KVM_TDX_INIT_MEM_REGION; ADD the page to the TD.
 	 */
-	if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE))
-		return tdx_mem_page_add(kvm, gfn, level, pfn);
+	if (likely(kvm_tdx->state == TD_STATE_RUNNABLE))
+		ret = tdx_mem_page_aug(kvm, gfn, level, pfn);
+	else
+		ret = tdx_mem_page_add(kvm, gfn, level, pfn);
 
-	return tdx_mem_page_aug(kvm, gfn, level, pfn);
+	if (ret)
+		tdx_pamt_put(pfn);
+
+	return ret;
 }
 
 /*
@@ -1822,6 +1863,7 @@ static int tdx_sept_remove_leaf_spte(struct kvm *kvm, gfn_t gfn,
 		return -EIO;
 
 	tdx_quirk_reset_paddr(PFN_PHYS(pfn), PAGE_SIZE);
+	tdx_pamt_put(pfn);
 	return 0;
 }
 
@@ -1865,6 +1907,8 @@ static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn, u64 old_spte,
  */
 static void tdx_sept_free_private_spt(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
+	struct page *sept_pt = virt_to_page(sp->external_spt);
+
 	/*
 	 * KVM doesn't (yet) zap page table pages in mirror page table while
 	 * TD is active, though guest pages mapped in mirror page table could be
@@ -1878,15 +1922,15 @@ static void tdx_sept_free_private_spt(struct kvm *kvm, struct kvm_mmu_page *sp)
 	 * the page to prevent the kernel from accessing the encrypted page.
 	 */
 	if (KVM_BUG_ON(is_hkid_assigned(to_kvm_tdx(kvm)), kvm) ||
-	    tdx_reclaim_page(virt_to_page(sp->external_spt)))
+	    tdx_reclaim_page(sept_pt))
 		goto out;
 
 	/*
-	 * Immediately free the S-EPT page because RCU-time free is unnecessary
-	 * after TDH.PHYMEM.PAGE.RECLAIM ensures there are no outstanding
-	 * readers.
+	 * Immediately free the S-EPT page as the TDX subsystem doesn't support
+	 * freeing pages from RCU callbacks, and more importantly because
+	 * TDH.PHYMEM.PAGE.RECLAIM ensures there are no outstanding readers.
 	 */
-	free_page((unsigned long)sp->external_spt);
+	tdx_free_control_page(sept_pt);
 out:
 	sp->external_spt = NULL;
 }
@@ -3486,6 +3530,10 @@ int __init tdx_hardware_setup(void)
 
 	vt_x86_ops.set_external_spte = tdx_sept_set_private_spte;
 	vt_x86_ops.free_external_spt = tdx_sept_free_private_spt;
+
+	if (tdx_supports_dynamic_pamt(tdx_sysinfo))
+		vt_x86_ops.topup_external_cache = tdx_topup_external_pamt_cache;
+
 	vt_x86_ops.protected_apic_has_interrupt = tdx_protected_apic_has_interrupt;
 	return 0;
 
diff --git a/arch/x86/kvm/vmx/tdx.h b/arch/x86/kvm/vmx/tdx.h
index ac8323a68b163..fd368e3ee060b 100644
--- a/arch/x86/kvm/vmx/tdx.h
+++ b/arch/x86/kvm/vmx/tdx.h
@@ -72,6 +72,8 @@ struct vcpu_tdx {
 
 	u64 map_gpa_next;
 	u64 map_gpa_end;
+
+	struct tdx_pamt_cache pamt_cache;
 };
 
 void tdh_vp_rd_failed(struct vcpu_tdx *tdx, char *uclass, u32 field, u64 err);
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 4d90d1a555135..7fd427174b8b6 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -2044,6 +2044,7 @@ bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo)
 	/* To be enabled when kernel is ready. */
 	return false;
 }
+EXPORT_SYMBOL_FOR_KVM(tdx_supports_dynamic_pamt);
 
 static struct page *tdx_alloc_page_pamt_cache(struct tdx_pamt_cache *cache)
 {
-- 
2.54.0

Re: [PATCH v9 08/11] KVM: TDX: Get/put PAMT pages when (un)mapping private memory

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] Transient TDX_OPERAND_BUSY errors during private memory mapping cause KVM to indefinitely retry the fault without restocking vCPU memory caches, leading to cache depletion and deterministic VM or host crash.
--

--- Patch [8]: [PATCH v9 08/11] KVM: TDX: Get/put PAMT pages when (un)mapping private memory ---
commit caefd7119dad264568b4b1abac0ec41595539508
Author: Kiryl Shutsemau <kas@kernel.org>
Subject: KVM: TDX: Get/put PAMT pages when (un)mapping private memory

This patch adds Dynamic PAMT support to KVM's S-EPT MMU by getting a PAMT
page when adding guest memory, and putting the page when removing it.
The kernel supplies the TDX module with a few pages that cover 2MB of host
physical memory.

> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 957078e8656ef..09573112540e9 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c

> [ ... ]

> @@ -1679,16 +1694,28 @@ static struct page *tdx_spte_to_sept_pt(struct kvm *kvm, gfn_t gfn,
>  static int tdx_sept_map_nonleaf_spte(struct kvm *kvm, gfn_t gfn,
>  				     enum pg_level level, u64 new_spte)
>  {
> +	struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
>  	gpa_t gpa = gfn_to_gpa(gfn);
>  	u64 err, entry, level_state;
>  	struct page *sept_pt;
> +	int ret;
> +
> +	if (KVM_BUG_ON(!vcpu, kvm))
> +		return -EIO;
>  
>  	sept_pt = tdx_spte_to_sept_pt(kvm, gfn, new_spte, level);
>  	if (!sept_pt)
>  		return -EIO;
>  
> +	ret = tdx_pamt_get(page_to_pfn(sept_pt), &to_tdx(vcpu)->pamt_cache);
> +	if (KVM_BUG_ON(ret, kvm))
> +		return ret;
> +
>  	err = tdh_mem_sept_add(&to_kvm_tdx(kvm)->td, gpa, level, sept_pt,
>  			       &entry, &level_state);
> +	if (err)
> +		tdx_pamt_put(page_to_pfn(sept_pt));
> +

[Severity: Critical]
When tdh_mem_sept_add() returns an error like TDX_OPERAND_BUSY, this path
calls tdx_pamt_put(), which frees the page back to the global allocator
instead of returning it to the vCPU cache.

Since a busy error returns -EBUSY and triggers a RET_PF_RETRY loop in
kvm_tdp_mmu_map_private_pfn():

kvm_tdp_mmu_map_private_pfn() {
    ...
    do {
        ...
        r = kvm_tdp_mmu_map(vcpu, &fault);
    } while (r == RET_PF_RETRY);
    ...
}

Does this repeatedly drain the vCPU's memory cache without topping it up?

The loop re-attempts the mapping without calling mmu_topup_memory_caches(),
meaning repeated contentions could quickly exhaust the cache capacity.

>  	if (unlikely(tdx_operand_busy(err)))
>  		return -EBUSY;
>  
> [ ... ]
> @@ -1701,8 +1728,13 @@ static int tdx_sept_map_nonleaf_spte(struct kvm *kvm, gfn_t gfn,
>  static int tdx_sept_map_leaf_spte(struct kvm *kvm, gfn_t gfn, enum pg_level level,
>  				  u64 new_spte)
>  {
> +	struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
>  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
>  	kvm_pfn_t pfn = spte_to_pfn(new_spte);
> +	int ret;
> +
> +	if (KVM_BUG_ON(!vcpu, kvm))
> +		return -EIO;
>  
>  	/* TODO: handle large pages. */
>  	if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
> @@ -1710,6 +1742,10 @@ static int tdx_sept_map_leaf_spte(struct kvm *kvm, gfn_t gfn, enum pg_level leve
>  
>  	WARN_ON_ONCE((new_spte & VMX_EPT_RWX_MASK) != VMX_EPT_RWX_MASK);
>  
> +	ret = tdx_pamt_get(pfn, &to_tdx(vcpu)->pamt_cache);
> +	if (KVM_BUG_ON(ret, kvm))
> +		return ret;
> +

[Severity: Critical]
If the vCPU's pamt_cache is exhausted due to the retry loop mentioned above,
tdx_pamt_get() will return -ENOMEM.

Does this KVM_BUG_ON() trigger when the cache depletes, permanently killing
the VM?

Could a busy guest intentionally induce TDX_OPERAND_BUSY contentions by
performing memory operations simultaneously with TDH.VP.ENTER on other vCPUs
to reliably trigger this crash?

>  	/*
>  	 * Ensure pre_fault_allowed is read by kvm_arch_vcpu_pre_fault_memory()
>  	 * before kvm_tdx->state.  Userspace must not be allowed to pre-fault

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260806020850.1221381-1-rick.p.edgecombe@intel.com?part=8

Re: [PATCH v9 08/11] KVM: TDX: Get/put PAMT pages when (un)mapping private memory

[Edgecombe, Rick P]

On Thu, 2026-08-06 at 07:16 +0000, sashiko-bot@kernel.org wrote:
> > [Severity: Critical]
> > When tdh_mem_sept_add() returns an error like TDX_OPERAND_BUSY, this path
> > calls tdx_pamt_put(), which frees the page back to the global allocator
> > instead of returning it to the vCPU cache.
> > 
> > Since a busy error returns -EBUSY and triggers a RET_PF_RETRY loop in
> > kvm_tdp_mmu_map_private_pfn():
> > 
> > kvm_tdp_mmu_map_private_pfn() {
> >     ...
> >     do {
> >         ...
> >         r = kvm_tdp_mmu_map(vcpu, &fault);
> >     } while (r == RET_PF_RETRY);
> >     ...
> > }
> > 
> > Does this repeatedly drain the vCPU's memory cache without topping it up?
> > 
> > The loop re-attempts the mapping without calling mmu_topup_memory_caches(),
> > meaning repeated contentions could quickly exhaust the cache capacity.

This is the same false positive we get every time. Existing issue. Sean was
going to send a cleanup. Oh he just did.

Re: [PATCH v9 08/11] KVM: TDX: Get/put PAMT pages when (un)mapping private memory

[Dave Hansen]

On 8/5/26 19:08, Rick Edgecombe wrote:
> Add Dynamic PAMT support to KVM's S-EPT MMU by "getting" a PAMT page when
> adding guest memory (PAGE.ADD or PAGE.AUG), and "putting" the page when
> removing guest memory (PAGE.REMOVE).

Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

[PATCH v9 09/11] x86/virt/tdx: Enable Dynamic PAMT

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

The Physical Address Metadata Table (PAMT) holds TDX metadata for
physical memory and must be allocated by the kernel during TDX module
initialization. Dynamic PAMT is a TDX module feature that can reduce this
memory use by allocating part of the PAMT dynamically.

The TDX module exposes whether Dynamic PAMT is supported via a bit in the
'features0' metadata. Unfortunately, the TDX module exposes the feature as
supported even when it does not support using it with the number of keyids
currently configured in the BIOS. Since no TDX modules exist today with
that issue fixed, make the feature default off to prevent users from
upgrading their kernel and encountering TDX erroring out when trying to
enable Dynamic PAMT.

For the decision of whether to make it a boot time option and/or compile
time option, consider that Dynamic PAMT's memory savings are significant
enough to make it a good default configuration. That is most TDX users
should want it unless they have strange keyid configurations.

The feature increases the kernel size by 2KB (when TDX is configured in the
build).

All pieces are in place to enable Dynamic PAMT if it is supported and the
user passes a kernel parameter.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
---
v8:
 - Order tdx_dpamt in kernel-parameters.txt (Sohil)
 - Make tdx_enable_dpamt static (Sashiko)

v7:
 - Add kernel parameter following some twists and turns, derriving
   originally from a comment by (Chao)
---
 .../admin-guide/kernel-parameters.txt         |  7 +++++++
 arch/x86/include/asm/tdx.h                    |  1 +
 arch/x86/virt/vmx/tdx/tdx.c                   | 21 +++++++++++++++++--
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b5493a7f8f228..49bcd7798876c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -7512,6 +7512,13 @@ Kernel parameters
 
 	tdfx=		[HW,DRM]
 
+	tdx_dpamt=
+			[X86] Controls whether TDX will use Dynamic PAMT
+			to save memory, when supported.
+
+			Valid parameters: "on", "off"
+			Default: "off"
+
 	test_suspend=	[SUSPEND]
 			Format: { "mem" | "standby" | "freeze" }[,N]
 			Specify "mem" (for Suspend-to-RAM) or "standby" (for
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 9cbd250bbd39b..7910901a7ba21 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -36,6 +36,7 @@
 /* Bit definitions of TDX_FEATURES0 metadata field */
 #define TDX_FEATURES0_TD_PRESERVING	BIT_ULL(1)
 #define TDX_FEATURES0_NO_RBP_MOD	BIT_ULL(18)
+#define TDX_FEATURES0_DYNAMIC_PAMT	BIT_ULL(36)
 
 #ifndef __ASSEMBLER__
 
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 7fd427174b8b6..f2ea034773bb2 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -47,6 +47,8 @@
 #include "seamcall_internal.h"
 #include "tdx.h"
 
+static bool tdx_enable_dpamt __ro_after_init;
+
 struct tdx_module_state {
 	bool initialized;
 	bool sysinit_done;
@@ -1028,6 +1030,8 @@ static __init int construct_tdmrs(struct list_head *tmb_list,
 	return ret;
 }
 
+#define TDX_SYS_CONFIG_DYNAMIC_PAMT	BIT(16)
+
 static __init int config_tdx_module(struct tdmr_info_list *tdmr_list,
 				    u64 global_keyid)
 {
@@ -1056,6 +1060,12 @@ static __init int config_tdx_module(struct tdmr_info_list *tdmr_list,
 	args.rcx = __pa(tdmr_pa_array);
 	args.rdx = tdmr_list->nr_consumed_tdmrs;
 	args.r8 = global_keyid;
+
+	if (tdx_supports_dynamic_pamt(&tdx_sysinfo)) {
+		pr_info("Enable Dynamic PAMT\n");
+		args.r8 |= TDX_SYS_CONFIG_DYNAMIC_PAMT;
+	}
+
 	ret = seamcall_prerr(TDH_SYS_CONFIG, &args);
 
 	/* Free the array as it is not required anymore. */
@@ -2041,8 +2051,8 @@ EXPORT_SYMBOL_FOR_KVM(tdh_phymem_page_wbinvd_hkid);
 
 bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo)
 {
-	/* To be enabled when kernel is ready. */
-	return false;
+	return sysinfo->features.tdx_features0 & TDX_FEATURES0_DYNAMIC_PAMT &&
+	       tdx_enable_dpamt;
 }
 EXPORT_SYMBOL_FOR_KVM(tdx_supports_dynamic_pamt);
 
@@ -2300,6 +2310,13 @@ void tdx_free_control_page(struct page *page)
 }
 EXPORT_SYMBOL_FOR_KVM(tdx_free_control_page);
 
+static int __init tdx_dpamt_setup(char *str)
+{
+	return kstrtobool(str, &tdx_enable_dpamt) == 0;
+}
+
+__setup("tdx_dpamt=", tdx_dpamt_setup);
+
 void tdx_sys_disable(void)
 {
 	struct tdx_module_args args = {};
-- 
2.54.0

[PATCH v9 10/11] Documentation/x86: Add documentation for TDX's Dynamic PAMT

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Expand TDX documentation to include information on the Dynamic PAMT
feature.

The new section explains PAMT support in the TDX module and how Dynamic
PAMT affects the kernel memory use.

AI was used under supervision to review the docs.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
---
v7:
 - Spell out PAMT acronym (Binbin)
 - Drop Assisted-by tag and cover AI use in log (Dave)
 - Add info about kernel parameter

v6:
 - Add missing word (Binbin)
 - Use "::" instead of ":"
 - Make format of dmesg example accurate
---
 .../admin-guide/kernel-parameters.txt         |  3 ++
 Documentation/arch/x86/tdx.rst                | 28 +++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 49bcd7798876c..a0a7670246bc5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -7519,6 +7519,9 @@ Kernel parameters
 			Valid parameters: "on", "off"
 			Default: "off"
 
+			For details see:
+			Documentation/arch/x86/tdx.rst
+
 	test_suspend=	[SUSPEND]
 			Format: { "mem" | "standby" | "freeze" }[,N]
 			Specify "mem" (for Suspend-to-RAM) or "standby" (for
diff --git a/Documentation/arch/x86/tdx.rst b/Documentation/arch/x86/tdx.rst
index 3303499ad4c6f..a1c2309230580 100644
--- a/Documentation/arch/x86/tdx.rst
+++ b/Documentation/arch/x86/tdx.rst
@@ -200,6 +200,34 @@ reflects the TCB of the currently running TDX module and therefore
 changes after an update. By contrast, TEE_TCB_SVN reflects the TCB at TD
 launch time and is not affected.
 
+Dynamic PAMT
+------------
+
+Physical Address Metadata Table (PAMT) is memory that the TDX module needs
+to keep data about each page (think like struct page). It needs to be handed
+to the TDX module for its exclusive use. For normal PAMT, this is installed
+when the TDX module is first loaded and comes to about 0.4% of system memory.
+
+Dynamic PAMT is a TDX module feature that allows VMM to allocate part of the
+PAMT as needed (the parts for tracking 4KB size pages). The other page sizes
+(1GB and 2MB) are still allocated statically at the time of TDX module
+initialization. This reduces the amount of memory that TDX uses while TDs are
+not in use.
+
+When Dynamic PAMT is in use, dmesg shows it like::
+
+  [..] virt/tdx: Enable Dynamic PAMT
+  [..] virt/tdx: 10092 KB allocated for PAMT
+  [..] virt/tdx: TDX-Module initialized
+
+Dynamic PAMT is only enabled when supported and the ``tdx_dpamt=`` kernel
+parameter is set to "on". The feature is off by default because TDX module
+internal details prevent Dynamic PAMT from working on all keyid partitioning
+configurations. When the TDX module is fixed to include these constraints in
+its enumeration of Dynamic PAMT support, kernel support can be changed to
+default on. For more information, consult the Intel TDX documentation about
+Dynamic PAMT.
+
 TDX Interaction to Other Kernel Components
 ------------------------------------------
 
-- 
2.54.0

[PATCH v9 11/11] x86/virt/tdx: Optimize tdx_pamt_get/put()

[Rick Edgecombe]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

The Dynamic PAMT get/put helpers use a global spinlock to serialize all
refcount updates and SEAMCALL invocations. This gives correct behavior for
concurrent callers, but leads to contention. It is especially bad from the
KVM side, which is designed to allow faulting in EPT under a shared lock.
With the global spinlock, not only is the lock an exclusive one, but it is
for all TDs instead of just a single one.

But taking the global lock each time is actually unnecessary. Only the 0->1
and 1->0 refcount transitions actually need the lock (to pair with
SEAMCALLs that actually add and remove with the Dynamic PAMT pages). The
common case of incrementing or decrementing a non-zero refcount can be
done locklessly.

So create a fast and slow path. Check the refcount outside the lock and
only take it for the slow path (0->1 and 1->0 transitions).

On the put side make the refcount adjustment and lock taking atomic so if
a 'get' happens between them, it doesn't cause the Dynamic PAMT to be
freed incorrectly. On the get side there is no technique for doing the
refcount adjustment and lock atomically, so check the refcount again
inside the lock.

AI was used under supervision to collect/apply feedback, review code and
workshop logs. It assisted in identifying/evaluating the stale
conditionals for the races resolved from the atomic_dec_and_lock() change.
Separate from atomic_dec_and_lock() fallout, it suggested to change
atomic_inc() to atomic_set(pamt_refcount, 1) in the put error path for the
sake of being more precise, which Kiryl had also suggested in the past.
The model also suggested updated comments following the
atomic_dec_and_lock() change based on some directed prompting. The
comments were subsequently edited or further prompted for fine tuning.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
---
v7:
 - Drop Assisted-by tag and cover AI use in log (Dave)
 - Move to end of the series
 - Use atomic_inc_not_zero() in this patch inside the spin_lock(), as
   suggested on non-optimized patch by. (Dave)

v6:
 - Fix "tdx_pamt_add()" typo to "tdx_pamt_get()" in lost-race comment
 - Fix error path bug: set ret = -EIO and use WARN_ON_ONCE() instead of
   pr_err() for unexpected PAMT.ADD failures (Sean)
 - Use "set the refcount 0->1" wording to match atomic_set() usage
 - Wrap comments to 80 columns
 - Switch to atomic_dec_and_lock() and remove handling of races that are
   no longer needed as a result. Adjust comments as appropriate. (Dave)
 - Adjustments from dropping error helper patches
---
 arch/x86/virt/vmx/tdx/tdx.c | 44 ++++++++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 15 deletions(-)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index f2ea034773bb2..dec45709e454a 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -2166,28 +2166,41 @@ int tdx_pamt_get(kvm_pfn_t pfn, struct tdx_pamt_cache *cache)
 	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
 		return 0;
 
-	ret = alloc_pamt_array(pamt_pages, cache);
-	if (ret)
-		return ret;
-
 	pamt_refcount = tdx_find_pamt_refcount(pfn);
 
-	spin_lock(&pamt_lock);
-
 	/*
 	 * If the pamt page is already added (i.e. refcount >= 1),
 	 * then just increment the refcount.
 	 */
+	if (atomic_inc_not_zero(pamt_refcount))
+		return 0;
+
+	ret = alloc_pamt_array(pamt_pages, cache);
+	if (ret)
+		return ret;
+
+	spin_lock(&pamt_lock);
+
+	/*
+	 * Unlike tdx_pamt_put() which uses atomic_dec_and_lock() to
+	 * atomically handle the 1->0 transition, the get side has no
+	 * equivalent combined primitive for 0->1. Recheck under the
+	 * lock since another get may have already done the 0->1
+	 * transition after both saw atomic_inc_not_zero() fail.
+	 */
 	if (atomic_inc_not_zero(pamt_refcount))
 		goto out_free;
 
-	/* Try to add the PAMT page and take the refcount 0->1. */
 	tdx_status = tdh_phymem_pamt_add(pfn, pamt_pages);
 	if (WARN_ON_ONCE(tdx_status != TDX_SUCCESS)) {
 		ret = -EIO;
 		goto out_free;
 	}
 
+	/*
+	 * The refcount is zero, and this locked path is the
+	 * only way to increase it from 0->1.
+	 */
 	atomic_set(pamt_refcount, 1);
 	spin_unlock(&pamt_lock);
 	return 0;
@@ -2212,17 +2225,13 @@ void tdx_pamt_put(kvm_pfn_t pfn)
 
 	pamt_refcount = tdx_find_pamt_refcount(pfn);
 
-	spin_lock(&pamt_lock);
 	/*
 	 * If there is more than 1 reference on the pamt page, don't
 	 * remove it yet. Just decrement the refcount.
 	 */
-	if (atomic_read(pamt_refcount) > 1) {
-		atomic_dec(pamt_refcount);
-		goto out_unlock;
-	}
+	if (!atomic_dec_and_lock(pamt_refcount, &pamt_lock))
+		return;
 
-	/* Try to remove the pamt page and take the refcount 1->0. */
 	tdx_status = tdh_phymem_pamt_remove(pfn, pamt_pages);
 
 	/*
@@ -2232,10 +2241,15 @@ void tdx_pamt_put(kvm_pfn_t pfn)
 	 * failure indicates a kernel bug, memory is being leaked, and
 	 * the dangling PAMT entry may cause future operations to fail.
 	 */
-	if (WARN_ON_ONCE(tdx_status != TDX_SUCCESS))
+	if (WARN_ON_ONCE(tdx_status != TDX_SUCCESS)) {
+		/*
+		 * atomic_dec_and_lock() already decremented it to 0,
+		 * but the PAMT entry still exists since REMOVE failed.
+		 */
+		atomic_set(pamt_refcount, 1);
 		goto out_unlock;
+	}
 
-	atomic_set(pamt_refcount, 0);
 	spin_unlock(&pamt_lock);
 	free_pamt_array(pamt_pages);
 	return;
-- 
2.54.0