[PATCH 03/28] KVM: x86/mmu: free up bit 10 of PTEs in preparation for MBEC

Paolo Bonzini
2026-04-30T15:07:59Z

From: Jon Kohler <jon@nutanix.com>

Update SPTE_MMIO_ALLOWED_MASK to allow EPT user executable (bit 10) to
be treated like EPT RWX bit2:0, as when mode-based execute control is
enabled, bit 10 can act like a "present" bit. Likewise do not include
it in FROZEN_SPTE.

No functional changes intended, other than the reduction of the maximum
MMIO generation that is stored in page tables.

Cc: Kai Huang <kai.huang@intel.com>
Signed-off-by: Jon Kohler <jon@nutanix.com>
Message-ID: <20251223054806.1611168-4-jon@nutanix.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Tested-by: David Riley <d.riley@proxmox.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/include/asm/vmx.h | 2 ++
arch/x86/kvm/mmu/spte.h | 20 +++++++++++---------
2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
index b2291a766e3f..2b30b921b375 100644
--- a/arch/x86/include/asm/vmx.h
+++ b/arch/x86/include/asm/vmx.h
@@ -560,10 +560,12 @@ enum vmcs_field {
#define VMX_EPT_ACCESS_BIT (1ull << 8)
#define VMX_EPT_DIRTY_BIT (1ull << 9)
#define VMX_EPT_SUPPRESS_VE_BIT (1ull << 63)
+
#define VMX_EPT_RWX_MASK (VMX_EPT_READABLE_MASK | \
VMX_EPT_WRITABLE_MASK | \
VMX_EPT_EXECUTABLE_MASK)
#define VMX_EPT_MT_MASK (7ull << VMX_EPT_MT_EPTE_SHIFT)
+#define VMX_EPT_USER_EXECUTABLE_MASK (1ull << 10)

static inline u8 vmx_eptp_page_walk_level(u64 eptp)
{
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 28086fa86fe0..4283cea3e66c 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -96,11 +96,11 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
#undef SHADOW_ACC_TRACK_SAVED_MASK

/*
- * Due to limited space in PTEs, the MMIO generation is a 19 bit subset of
+ * Due to limited space in PTEs, the MMIO generation is an 18 bit subset of
* the memslots generation and is derived as follows:
*
- * Bits 0-7 of the MMIO generation are propagated to spte bits 3-10
- * Bits 8-18 of the MMIO generation are propagated to spte bits 52-62
+ * Bits 0-6 of the MMIO generation are propagated to spte bits 3-9
+ * Bits 7-17 of the MMIO generation are propagated to spte bits 52-62
*
* The KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS flag is intentionally not included in
* the MMIO generation number, as doing so would require stealing a bit from
@@ -111,7 +111,7 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
*/

#define MMIO_SPTE_GEN_LOW_START 3
-#define MMIO_SPTE_GEN_LOW_END 10
+#define MMIO_SPTE_GEN_LOW_END 9

#define MMIO_SPTE_GEN_HIGH_START 52
#define MMIO_SPTE_GEN_HIGH_END 62
@@ -133,7 +133,8 @@ static_assert(!(SPTE_MMU_PRESENT_MASK &
* and so they're off-limits for generation; additional checks ensure the mask
* doesn't overlap legal PA bits), and bit 63 (carved out for future usage).
*/
-#define SPTE_MMIO_ALLOWED_MASK (BIT_ULL(63) | GENMASK_ULL(51, 12) | GENMASK_ULL(2, 0))
+#define SPTE_MMIO_ALLOWED_MASK (BIT_ULL(63) | GENMASK_ULL(51, 12) | \
+ BIT_ULL(10) | GENMASK_ULL(2, 0))
static_assert(!(SPTE_MMIO_ALLOWED_MASK &
(SPTE_MMU_PRESENT_MASK | MMIO_SPTE_GEN_LOW_MASK | MMIO_SPTE_GEN_HIGH_MASK)));

@@ -141,7 +142,7 @@ static_assert(!(SPTE_MMIO_ALLOWED_MASK &
#define MMIO_SPTE_GEN_HIGH_BITS (MMIO_SPTE_GEN_HIGH_END - MMIO_SPTE_GEN_HIGH_START + 1)

/* remember to adjust the comment above as well if you change these */
-static_assert(MMIO_SPTE_GEN_LOW_BITS == 8 && MMIO_SPTE_GEN_HIGH_BITS == 11);
+static_assert(MMIO_SPTE_GEN_LOW_BITS == 7 && MMIO_SPTE_GEN_HIGH_BITS == 11);

#define MMIO_SPTE_GEN_LOW_SHIFT (MMIO_SPTE_GEN_LOW_START - 0)
#define MMIO_SPTE_GEN_HIGH_SHIFT (MMIO_SPTE_GEN_HIGH_START - MMIO_SPTE_GEN_LOW_BITS)
@@ -217,10 +218,11 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
*
* Only used by the TDP MMU.
*/
-#define FROZEN_SPTE (SHADOW_NONPRESENT_VALUE | 0x5a0ULL)
+#define FROZEN_SPTE (SHADOW_NONPRESENT_VALUE | 0x1a0ULL)

-/* Frozen SPTEs must not be misconstrued as shadow present PTEs. */
-static_assert(!(FROZEN_SPTE & SPTE_MMU_PRESENT_MASK));
+/* Frozen SPTEs must not be misconstrued as shadow or MMU present PTEs. */
+static_assert(!(FROZEN_SPTE & (SPTE_MMU_PRESENT_MASK |
+ VMX_EPT_RWX_MASK | VMX_EPT_USER_EXECUTABLE_MASK)));

static inline bool is_frozen_spte(u64 spte)
{
--
2.52.0