Use low "available" bits to tag REMOVED SPTEs. Using a high bit is
moderately costly as it often causes the compiler to generate a 64-bit
immediate. More importantly, this makes it very clear REMOVED_SPTE is
a value, not a flag.
Cc: Ben Gardon <bgardon@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
arch/x86/kvm/mmu/spte.c | 11 ++++++++++-
arch/x86/kvm/mmu/spte.h | 11 +++++++----
2 files changed, 17 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index e07aabb23b8a..66d43cec0c31 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -277,7 +277,16 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
SHADOW_NONPRESENT_OR_RSVD_MASK_LEN)))
mmio_value = 0;
- WARN_ON((mmio_value & mmio_mask) != mmio_value);
+ /*
+ * The masked MMIO value must obviously match itself and a removed SPTE
+ * must not get a false positive. Removed SPTEs and MMIO SPTEs should
+ * never collide as MMIO must set some RWX bits, and removed SPTEs must
+ * not set any RWX bits.
+ */
+ if (WARN_ON((mmio_value & mmio_mask) != mmio_value) ||
+ WARN_ON(mmio_value && (REMOVED_SPTE & mmio_mask) == mmio_value))
+ mmio_value = 0;
+
shadow_mmio_value = mmio_value;
shadow_mmio_mask = mmio_mask;
shadow_mmio_access_mask = access_mask;
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 2fad4ccd3679..b53036d9ddf3 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -174,13 +174,16 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
* non-present intermediate value. Other threads which encounter this value
* should not modify the SPTE.
*
- * This constant works because it is considered non-present on both AMD and
- * Intel CPUs and does not create a L1TF vulnerability because the pfn section
- * is zeroed out.
+ * Use a semi-arbitrary value that doesn't set RWX bits, i.e. is not-present on
+ * bot AMD and Intel CPUs, and doesn't set PFN bits, i.e. doesn't create a L1TF
+ * vulnerability. Use only low bits to avoid 64-bit immediates.
*
* Only used by the TDP MMU.
*/
-#define REMOVED_SPTE (1ull << 59)
+#define REMOVED_SPTE 0x5a0ULL
+
+/* Removed SPTEs must not be misconstrued as shadow present PTEs. */
+static_assert(!(REMOVED_SPTE & SPTE_MMU_PRESENT_MASK));
static inline bool is_removed_spte(u64 spte)
{
--
2.30.1.766.gb4fecdf3b7-goog
