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KVM: x86/mmu: Use low available bits for removed SPTEs

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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@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-24-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson
2021-02-25 12:47:48 -08:00
committed by Paolo Bonzini
parent 7a51393ae0
commit 715f1079ee
2 changed files with 17 additions and 5 deletions
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11
arch/x86/kvm/mmu/spte.c
View File

@@ -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;

11
arch/x86/kvm/mmu/spte.h
View File

@@ -176,13 +176,16 @@ 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)
{