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KVM: x86/mmu: Optimize not-present/MMIO SPTE check in get_mmio_spte() 

Check only the terminal leaf for a "!PRESENT || MMIO" SPTE when looking
for reserved bits on valid, non-MMIO SPTEs.  The get_walk() helpers
terminate their walks if a not-present or MMIO SPTE is encountered, i.e.
the non-terminal SPTEs have already been verified to be regular SPTEs.
This eliminates an extra check-and-branch in a relatively hot loop.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20201218003139.2167891-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2020-12-17 16:31:39 -08:00 committed by Paolo Bonzini
parent dde81f9477
commit 9aa418792f
1 changed files with 13 additions and 7 deletions
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20
arch/x86/kvm/mmu/mmu.c
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@ -3511,7 +3511,7 @@ static int get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes, int *root_level
return leaf;
}
/* return true if reserved bit is detected on spte. */
/* return true if reserved bit(s) are detected on a valid, non-MMIO SPTE. */
static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
{
u64 sptes[PT64_ROOT_MAX_LEVEL + 1];
@ -3534,11 +3534,20 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
return reserved;
}
*sptep = sptes[leaf];
/*
* Skip reserved bits checks on the terminal leaf if it's not a valid
* SPTE. Note, this also (intentionally) skips MMIO SPTEs, which, by
* design, always have reserved bits set. The purpose of the checks is
* to detect reserved bits on non-MMIO SPTEs. i.e. buggy SPTEs.
*/
if (!is_shadow_present_pte(sptes[leaf]))
leaf++;
rsvd_check = &vcpu->arch.mmu->shadow_zero_check;
for (level = root; level >= leaf; level--) {
if (!is_shadow_present_pte(sptes[level]))
break;
for (level = root; level >= leaf; level--)
/*
* Use a bitwise-OR instead of a logical-OR to aggregate the
* reserved bit and EPT's invalid memtype/XWR checks to avoid
@ -3546,7 +3555,6 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
*/
reserved |= __is_bad_mt_xwr(rsvd_check, sptes[level]) |
__is_rsvd_bits_set(rsvd_check, sptes[level], level);
}
if (reserved) {
pr_err("%s: detect reserved bits on spte, addr 0x%llx, dump hierarchy:\n",
@ -3556,8 +3564,6 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
sptes[level], level);
}
*sptep = sptes[leaf];
return reserved;
}