[PATCH V5 0/3] x86/tdx: Skip clearing reclaimed pages unless X86_BUG_TDX_PW_MCE is present

[PATCH V5 0/3] x86/tdx: Skip clearing reclaimed pages unless X86_BUG_TDX_PW_MCE is present

[Adrian Hunter]

Hi

Here are 3 small self-explanatory patches related to clearing TDX private
pages.

Patch 1 and 2 are minor tidy-ups.

In patch 3, by skipping the clearing step, shutdown time can improve by
up to 40%.


Changes in V5:

      x86/tdx: Tidy reset_pamt functions
	New patch

Changes in V4:

      x86/tdx: Eliminate duplicate code in tdx_clear_page()
	Add and use tdx_quirk_reset_page() for KVM (Sean)

      x86/tdx: Skip clearing reclaimed pages unless X86_BUG_TDX_PW_MCE is present
	Add TDX Module Base spec. version (Rick)
	Add Rick's Rev'd-by

Changes in V3:

      x86/tdx: Eliminate duplicate code in tdx_clear_page()
	Explain "quirk" rename in commit message (Rick)
	Explain mb() change in commit message  (Rick)
	Add Rev'd-by, Ack'd-by tags

      x86/tdx: Skip clearing reclaimed pages unless X86_BUG_TDX_PW_MCE is present
	Remove "flush cache" comments (Rick)
	Update function comment to better relate to "quirk" naming (Rick)
	Add "via MOVDIR64B" to comment (Xiaoyao)
	Add Rev'd-by, Ack'd-by tags

Changes in V2 (as requested by Dave):

      x86/tdx: Eliminate duplicate code in tdx_clear_page()
	Rename reset_tdx_pages() to tdx_quirk_reset_paddr()
	Call tdx_quirk_reset_paddr() directly

      x86/tdx: Skip clearing reclaimed pages unless X86_BUG_TDX_PW_MCE is present
	Improve the comment


Adrian Hunter (3):
      x86/tdx: Eliminate duplicate code in tdx_clear_page()
      x86/tdx: Tidy reset_pamt functions
      x86/tdx: Skip clearing reclaimed pages unless X86_BUG_TDX_PW_MCE is present

 arch/x86/include/asm/tdx.h  |  2 ++
 arch/x86/kvm/vmx/tdx.c      | 25 +++----------------------
 arch/x86/virt/vmx/tdx/tdx.c | 36 +++++++++++++++++++-----------------
 3 files changed, 24 insertions(+), 39 deletions(-)


Regards
Adrian

[PATCH V5 1/3] x86/tdx: Eliminate duplicate code in tdx_clear_page()

[Adrian Hunter]

tdx_clear_page() and reset_tdx_pages() duplicate the TDX page clearing
logic.  Rename reset_tdx_pages() to tdx_quirk_reset_paddr() and create
tdx_quirk_reset_page() to call tdx_quirk_reset_paddr() and be used in
place of tdx_clear_page().

The new name reflects that, in fact, the clearing is necessary only for
hardware with a certain quirk.  That is dealt with in a subsequent patch
but doing the rename here avoids additional churn.

Note reset_tdx_pages() is slightly different from tdx_clear_page() because,
more appropriately, it uses mb() in place of __mb().  Except when extra
debugging is enabled (kcsan at present), mb() just calls __mb().

Reviewed-by: Kirill A. Shutemov <kas@kernel.org>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---


Changes in V5:

	None

Changes in V4:

	Add and use tdx_quirk_reset_page() for KVM (Sean)

Changes in V3:

	Explain "quirk" rename in commit message (Rick)
	Explain mb() change in commit message  (Rick)
	Add Rev'd-by, Ack'd-by tags

Changes in V2:

	Rename reset_tdx_pages() to tdx_quirk_reset_paddr()
	Call tdx_quirk_reset_paddr() directly


 arch/x86/include/asm/tdx.h  |  2 ++
 arch/x86/kvm/vmx/tdx.c      | 25 +++----------------------
 arch/x86/virt/vmx/tdx/tdx.c | 10 ++++++++--
 3 files changed, 13 insertions(+), 24 deletions(-)

diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 7ddef3a69866..57b46f05ff97 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -131,6 +131,8 @@ int tdx_guest_keyid_alloc(void);
 u32 tdx_get_nr_guest_keyids(void);
 void tdx_guest_keyid_free(unsigned int keyid);
 
+void tdx_quirk_reset_page(struct page *page);
+
 struct tdx_td {
 	/* TD root structure: */
 	struct page *tdr_page;
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 573d6f7d1694..ebb36229c7c8 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -283,25 +283,6 @@ static inline void tdx_disassociate_vp(struct kvm_vcpu *vcpu)
 	vcpu->cpu = -1;
 }
 
-static void tdx_clear_page(struct page *page)
-{
-	const void *zero_page = (const void *) page_to_virt(ZERO_PAGE(0));
-	void *dest = page_to_virt(page);
-	unsigned long i;
-
-	/*
-	 * The page could have been poisoned.  MOVDIR64B also clears
-	 * the poison bit so the kernel can safely use the page again.
-	 */
-	for (i = 0; i < PAGE_SIZE; i += 64)
-		movdir64b(dest + i, zero_page);
-	/*
-	 * MOVDIR64B store uses WC buffer.  Prevent following memory reads
-	 * from seeing potentially poisoned cache.
-	 */
-	__mb();
-}
-
 static void tdx_no_vcpus_enter_start(struct kvm *kvm)
 {
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
@@ -347,7 +328,7 @@ static int tdx_reclaim_page(struct page *page)
 
 	r = __tdx_reclaim_page(page);
 	if (!r)
-		tdx_clear_page(page);
+		tdx_quirk_reset_page(page);
 	return r;
 }
 
@@ -596,7 +577,7 @@ static void tdx_reclaim_td_control_pages(struct kvm *kvm)
 		pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err);
 		return;
 	}
-	tdx_clear_page(kvm_tdx->td.tdr_page);
+	tdx_quirk_reset_page(kvm_tdx->td.tdr_page);
 
 	__free_page(kvm_tdx->td.tdr_page);
 	kvm_tdx->td.tdr_page = NULL;
@@ -1717,7 +1698,7 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
 		pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err);
 		return -EIO;
 	}
-	tdx_clear_page(page);
+	tdx_quirk_reset_page(page);
 	tdx_unpin(kvm, page);
 	return 0;
 }
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index c7a9a087ccaf..fc8d8e444f15 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -637,7 +637,7 @@ static int tdmrs_set_up_pamt_all(struct tdmr_info_list *tdmr_list,
  * clear these pages.  Note this function doesn't flush cache of
  * these TDX private pages.  The caller should make sure of that.
  */
-static void reset_tdx_pages(unsigned long base, unsigned long size)
+static void tdx_quirk_reset_paddr(unsigned long base, unsigned long size)
 {
 	const void *zero_page = (const void *)page_address(ZERO_PAGE(0));
 	unsigned long phys, end;
@@ -654,9 +654,15 @@ static void reset_tdx_pages(unsigned long base, unsigned long size)
 	mb();
 }
 
+void tdx_quirk_reset_page(struct page *page)
+{
+	tdx_quirk_reset_paddr(page_to_phys(page), PAGE_SIZE);
+}
+EXPORT_SYMBOL_GPL(tdx_quirk_reset_page);
+
 static void tdmr_reset_pamt(struct tdmr_info *tdmr)
 {
-	tdmr_do_pamt_func(tdmr, reset_tdx_pages);
+	tdmr_do_pamt_func(tdmr, tdx_quirk_reset_paddr);
 }
 
 static void tdmrs_reset_pamt_all(struct tdmr_info_list *tdmr_list)
-- 
2.48.1

[PATCH V5 2/3] x86/tdx: Tidy reset_pamt functions

[Adrian Hunter]

tdx_quirk_reset_paddr() was renamed to reflect that, in fact, the clearing
is necessary only for hardware with a certain quirk.  That is dealt with in
a subsequent patch.

Rename reset_pamt functions to contain "quirk" to reflect the new
functionality, and remove the now misleading comment.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---


Changes in V5:

	New patch


 arch/x86/virt/vmx/tdx/tdx.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index fc8d8e444f15..9e4638f68ba0 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -660,17 +660,17 @@ void tdx_quirk_reset_page(struct page *page)
 }
 EXPORT_SYMBOL_GPL(tdx_quirk_reset_page);
 
-static void tdmr_reset_pamt(struct tdmr_info *tdmr)
+static void tdmr_quirk_reset_pamt(struct tdmr_info *tdmr)
 {
 	tdmr_do_pamt_func(tdmr, tdx_quirk_reset_paddr);
 }
 
-static void tdmrs_reset_pamt_all(struct tdmr_info_list *tdmr_list)
+static void tdmrs_quirk_reset_pamt_all(struct tdmr_info_list *tdmr_list)
 {
 	int i;
 
 	for (i = 0; i < tdmr_list->nr_consumed_tdmrs; i++)
-		tdmr_reset_pamt(tdmr_entry(tdmr_list, i));
+		tdmr_quirk_reset_pamt(tdmr_entry(tdmr_list, i));
 }
 
 static unsigned long tdmrs_count_pamt_kb(struct tdmr_info_list *tdmr_list)
@@ -1142,15 +1142,7 @@ static int init_tdx_module(void)
 	 * to the kernel.
 	 */
 	wbinvd_on_all_cpus();
-	/*
-	 * According to the TDX hardware spec, if the platform
-	 * doesn't have the "partial write machine check"
-	 * erratum, any kernel read/write will never cause #MC
-	 * in kernel space, thus it's OK to not convert PAMTs
-	 * back to normal.  But do the conversion anyway here
-	 * as suggested by the TDX spec.
-	 */
-	tdmrs_reset_pamt_all(&tdx_tdmr_list);
+	tdmrs_quirk_reset_pamt_all(&tdx_tdmr_list);
 err_free_pamts:
 	tdmrs_free_pamt_all(&tdx_tdmr_list);
 err_free_tdmrs:
-- 
2.48.1

[PATCH V5 3/3] x86/tdx: Skip clearing reclaimed pages unless X86_BUG_TDX_PW_MCE is present

[Adrian Hunter]

Avoid clearing reclaimed TDX private pages unless the platform is affected
by the X86_BUG_TDX_PW_MCE erratum. This significantly reduces VM shutdown
time on unaffected systems.

Background

KVM currently clears reclaimed TDX private pages using MOVDIR64B, which:

   - Clears the TD Owner bit (which identifies TDX private memory) and
     integrity metadata without triggering integrity violations.
   - Clears poison from cache lines without consuming it, avoiding MCEs on
     access (refer TDX Module Base spec. 1348549-006US section 6.5.
     Handling Machine Check Events during Guest TD Operation).

The TDX module also uses MOVDIR64B to initialize private pages before use.
If cache flushing is needed, it sets TDX_FEATURES.CLFLUSH_BEFORE_ALLOC.
However, KVM currently flushes unconditionally, refer commit 94c477a751c7b
("x86/virt/tdx: Add SEAMCALL wrappers to add TD private pages")

In contrast, when private pages are reclaimed, the TDX Module handles
flushing via the TDH.PHYMEM.CACHE.WB SEAMCALL.

Problem

Clearing all private pages during VM shutdown is costly. For guests
with a large amount of memory it can take minutes.

Solution

TDX Module Base Architecture spec. documents that private pages reclaimed
from a TD should be initialized using MOVDIR64B, in order to avoid
integrity violation or TD bit mismatch detection when later being read
using a shared HKID, refer April 2025 spec. "Page Initialization" in
section "8.6.2. Platforms not Using ACT: Required Cache Flush and
Initialization by the Host VMM"

That is an overstatement and will be clarified in coming versions of the
spec. In fact, as outlined in "Table 16.2: Non-ACT Platforms Checks on
Memory" and "Table 16.3: Non-ACT Platforms Checks on Memory Reads in Li
Mode" in the same spec, there is no issue accessing such reclaimed pages
using a shared key that does not have integrity enabled. Linux always uses
KeyID 0 which never has integrity enabled. KeyID 0 is also the TME KeyID
which disallows integrity, refer "TME Policy/Encryption Algorithm" bit
description in "Intel Architecture Memory Encryption Technologies" spec
version 1.6 April 2025. So there is no need to clear pages to avoid
integrity violations.

There remains a risk of poison consumption. However, in the context of
TDX, it is expected that there would be a machine check associated with the
original poisoning. On some platforms that results in a panic. However
platforms may support "SEAM_NR" Machine Check capability, in which case
Linux machine check handler marks the page as poisoned, which prevents it
from being allocated anymore, refer commit 7911f145de5fe ("x86/mce:
Implement recovery for errors in TDX/SEAM non-root mode")

Improvement

By skipping the clearing step on unaffected platforms, shutdown time
can improve by up to 40%.

On platforms with the X86_BUG_TDX_PW_MCE erratum (SPR and EMR), continue
clearing because these platforms may trigger poison on partial writes to
previously-private pages, even with KeyID 0, refer commit 1e536e1068970
("x86/cpu: Detect TDX partial write machine check erratum")

Reviewed-by: Kirill A. Shutemov <kas@kernel.org>
Acked-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---


Changes in V5:

	None

Changes in V4:

	Add TDX Module Base spec. version (Rick)
	Add Rick's Rev'd-by

Changes in V3:

	Remove "flush cache" comments (Rick)
	Update function comment to better relate to "quirk" naming (Rick)
	Add "via MOVDIR64B" to comment (Xiaoyao)
	Add Rev'd-by, Ack'd-by tags

Changes in V2:

	Improve the comment


 arch/x86/virt/vmx/tdx/tdx.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 9e4638f68ba0..823850399bb7 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -633,15 +633,19 @@ static int tdmrs_set_up_pamt_all(struct tdmr_info_list *tdmr_list,
 }
 
 /*
- * Convert TDX private pages back to normal by using MOVDIR64B to
- * clear these pages.  Note this function doesn't flush cache of
- * these TDX private pages.  The caller should make sure of that.
+ * Convert TDX private pages back to normal by using MOVDIR64B to clear these
+ * pages. Typically, any write to the page will convert it from TDX private back
+ * to normal kernel memory. Systems with the X86_BUG_TDX_PW_MCE erratum need to
+ * do the conversion explicitly via MOVDIR64B.
  */
 static void tdx_quirk_reset_paddr(unsigned long base, unsigned long size)
 {
 	const void *zero_page = (const void *)page_address(ZERO_PAGE(0));
 	unsigned long phys, end;
 
+	if (!boot_cpu_has_bug(X86_BUG_TDX_PW_MCE))
+		return;
+
 	end = base + size;
 	for (phys = base; phys < end; phys += 64)
 		movdir64b(__va(phys), zero_page);
-- 
2.48.1

Re: [PATCH V5 1/3] x86/tdx: Eliminate duplicate code in tdx_clear_page()

[Adrian Hunter]

On 24/07/2025 15:48, Adrian Hunter wrote:
> tdx_clear_page() and reset_tdx_pages() duplicate the TDX page clearing
> logic.  Rename reset_tdx_pages() to tdx_quirk_reset_paddr() and create
> tdx_quirk_reset_page() to call tdx_quirk_reset_paddr() and be used in
> place of tdx_clear_page().
> 
> The new name reflects that, in fact, the clearing is necessary only for
> hardware with a certain quirk.  That is dealt with in a subsequent patch
> but doing the rename here avoids additional churn.
> 
> Note reset_tdx_pages() is slightly different from tdx_clear_page() because,
> more appropriately, it uses mb() in place of __mb().  Except when extra
> debugging is enabled (kcsan at present), mb() just calls __mb().
> 
> Reviewed-by: Kirill A. Shutemov <kas@kernel.org>
> Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Acked-by: Kai Huang <kai.huang@intel.com>

Forgot to add Sean's Ack from V4

> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> ---
> 
> 
> Changes in V5:
> 
> 	None
> 
> Changes in V4:
> 
> 	Add and use tdx_quirk_reset_page() for KVM (Sean)
> 
> Changes in V3:
> 
> 	Explain "quirk" rename in commit message (Rick)
> 	Explain mb() change in commit message  (Rick)
> 	Add Rev'd-by, Ack'd-by tags
> 
> Changes in V2:
> 
> 	Rename reset_tdx_pages() to tdx_quirk_reset_paddr()
> 	Call tdx_quirk_reset_paddr() directly
> 
> 
>  arch/x86/include/asm/tdx.h  |  2 ++
>  arch/x86/kvm/vmx/tdx.c      | 25 +++----------------------
>  arch/x86/virt/vmx/tdx/tdx.c | 10 ++++++++--
>  3 files changed, 13 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index 7ddef3a69866..57b46f05ff97 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -131,6 +131,8 @@ int tdx_guest_keyid_alloc(void);
>  u32 tdx_get_nr_guest_keyids(void);
>  void tdx_guest_keyid_free(unsigned int keyid);
>  
> +void tdx_quirk_reset_page(struct page *page);
> +
>  struct tdx_td {
>  	/* TD root structure: */
>  	struct page *tdr_page;
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 573d6f7d1694..ebb36229c7c8 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -283,25 +283,6 @@ static inline void tdx_disassociate_vp(struct kvm_vcpu *vcpu)
>  	vcpu->cpu = -1;
>  }
>  
> -static void tdx_clear_page(struct page *page)
> -{
> -	const void *zero_page = (const void *) page_to_virt(ZERO_PAGE(0));
> -	void *dest = page_to_virt(page);
> -	unsigned long i;
> -
> -	/*
> -	 * The page could have been poisoned.  MOVDIR64B also clears
> -	 * the poison bit so the kernel can safely use the page again.
> -	 */
> -	for (i = 0; i < PAGE_SIZE; i += 64)
> -		movdir64b(dest + i, zero_page);
> -	/*
> -	 * MOVDIR64B store uses WC buffer.  Prevent following memory reads
> -	 * from seeing potentially poisoned cache.
> -	 */
> -	__mb();
> -}
> -
>  static void tdx_no_vcpus_enter_start(struct kvm *kvm)
>  {
>  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> @@ -347,7 +328,7 @@ static int tdx_reclaim_page(struct page *page)
>  
>  	r = __tdx_reclaim_page(page);
>  	if (!r)
> -		tdx_clear_page(page);
> +		tdx_quirk_reset_page(page);
>  	return r;
>  }
>  
> @@ -596,7 +577,7 @@ static void tdx_reclaim_td_control_pages(struct kvm *kvm)
>  		pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err);
>  		return;
>  	}
> -	tdx_clear_page(kvm_tdx->td.tdr_page);
> +	tdx_quirk_reset_page(kvm_tdx->td.tdr_page);
>  
>  	__free_page(kvm_tdx->td.tdr_page);
>  	kvm_tdx->td.tdr_page = NULL;
> @@ -1717,7 +1698,7 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
>  		pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err);
>  		return -EIO;
>  	}
> -	tdx_clear_page(page);
> +	tdx_quirk_reset_page(page);
>  	tdx_unpin(kvm, page);
>  	return 0;
>  }
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index c7a9a087ccaf..fc8d8e444f15 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -637,7 +637,7 @@ static int tdmrs_set_up_pamt_all(struct tdmr_info_list *tdmr_list,
>   * clear these pages.  Note this function doesn't flush cache of
>   * these TDX private pages.  The caller should make sure of that.
>   */
> -static void reset_tdx_pages(unsigned long base, unsigned long size)
> +static void tdx_quirk_reset_paddr(unsigned long base, unsigned long size)
>  {
>  	const void *zero_page = (const void *)page_address(ZERO_PAGE(0));
>  	unsigned long phys, end;
> @@ -654,9 +654,15 @@ static void reset_tdx_pages(unsigned long base, unsigned long size)
>  	mb();
>  }
>  
> +void tdx_quirk_reset_page(struct page *page)
> +{
> +	tdx_quirk_reset_paddr(page_to_phys(page), PAGE_SIZE);
> +}
> +EXPORT_SYMBOL_GPL(tdx_quirk_reset_page);
> +
>  static void tdmr_reset_pamt(struct tdmr_info *tdmr)
>  {
> -	tdmr_do_pamt_func(tdmr, reset_tdx_pages);
> +	tdmr_do_pamt_func(tdmr, tdx_quirk_reset_paddr);
>  }
>  
>  static void tdmrs_reset_pamt_all(struct tdmr_info_list *tdmr_list)