[PATCH v3 15/16] x86/virt/tdx: Enable Dynamic PAMT

Rick Edgecombe
2025-09-18T23:23:11Z

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.

The exact size of the required PAMT memory is determined by the TDX
module and may vary between TDX module versions, but 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.

The Dynamic PAMT feature reduces static PAMT allocations. PAMT_1G and
PAMT_2M levels are still allocated on TDX module initialization, but the
PAMT_4K level is allocated dynamically, reducing static allocations to
approximately 0.004% of the system memory.

All pieces are in place. Enable Dynamic PAMT if it is supported.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
v3:
- No changes
---
arch/x86/include/asm/tdx.h | 6 +++++-
arch/x86/virt/vmx/tdx/tdx.c | 8 ++++++++
arch/x86/virt/vmx/tdx/tdx.h | 3 ---
3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index e108b48af2c3..d389dc07c718 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -12,6 +12,10 @@
#include <asm/trapnr.h>
#include <asm/shared/tdx.h>

+/* Bit definitions of TDX_FEATURES0 metadata field */
+#define TDX_FEATURES0_NO_RBP_MOD BIT_ULL(18)
+#define TDX_FEATURES0_DYNAMIC_PAMT BIT_ULL(36)
+
#ifndef __ASSEMBLER__

#include <uapi/asm/mce.h>
@@ -110,7 +114,7 @@ const struct tdx_sys_info *tdx_get_sysinfo(void);

static inline bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo)
{
- return false; /* To be enabled when kernel is ready */
+ return sysinfo->features.tdx_features0 & TDX_FEATURES0_DYNAMIC_PAMT;
}

int tdx_guest_keyid_alloc(void);
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index b4edc3ee495c..da21104575a6 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -1043,6 +1043,8 @@ static int construct_tdmrs(struct list_head *tmb_list,
return ret;
}

+#define TDX_SYS_CONFIG_DYNAMIC_PAMT BIT(16)
+
static int config_tdx_module(struct tdmr_info_list *tdmr_list, u64 global_keyid)
{
struct tdx_module_args args = {};
@@ -1070,6 +1072,12 @@ static int config_tdx_module(struct tdmr_info_list *tdmr_list, u64 global_keyid)
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. */
diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
index 46c4214b79fb..096c78a1d438 100644
--- a/arch/x86/virt/vmx/tdx/tdx.h
+++ b/arch/x86/virt/vmx/tdx/tdx.h
@@ -86,9 +86,6 @@ struct tdmr_info {
DECLARE_FLEX_ARRAY(struct tdmr_reserved_area, reserved_areas);
} __packed __aligned(TDMR_INFO_ALIGNMENT);

-/* Bit definitions of TDX_FEATURES0 metadata field */
-#define TDX_FEATURES0_NO_RBP_MOD BIT(18)
-
/*
* Do not put any hardware-defined TDX structure representations below
* this comment!
--
2.51.0