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 <[email protected]> Signed-off-by: Sean Christopherson <[email protected]> --- 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
