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KVM: x86/mmu: Drop exec/NX check from "page fault can be fast" 

Tweak the "page fault can be fast" logic to explicitly check for !PRESENT
faults in the access tracking case, and drop the exec/NX check that
becomes redundant as a result.  No sane hardware will generate an access
that is both an instruct fetch and a write, i.e. it's a waste of cycles.
If hardware goes off the rails, or KVM runs under a misguided hypervisor,
spuriously running throught fast path is benign (KVM has been uknowingly
being doing exactly that for years).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220423034752.1161007-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2022-04-23 03:47:45 +00:00 committed by Paolo Bonzini
parent 54275f74cf
commit 5c64aba517
1 changed files with 12 additions and 7 deletions
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19
arch/x86/kvm/mmu/mmu.c
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@ -3044,16 +3044,14 @@ static bool handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fa
static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
{
/*
* Do not fix the mmio spte with invalid generation number which
* need to be updated by slow page fault path.
* Page faults with reserved bits set, i.e. faults on MMIO SPTEs, only
* reach the common page fault handler if the SPTE has an invalid MMIO
* generation number. Refreshing the MMIO generation needs to go down
* the slow path. Note, EPT Misconfigs do NOT set the PRESENT flag!
*/
if (fault->rsvd)
return false;
/* See if the page fault is due to an NX violation */
if (unlikely(fault->exec && fault->present))
return false;
/*
* #PF can be fast if:
*
@ -3069,7 +3067,14 @@ static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
* SPTE is MMU-writable (determined later), the fault can be fixed
* by setting the Writable bit, which can be done out of mmu_lock.
*/
return !kvm_ad_enabled() || (fault->write && fault->present);
if (!fault->present)
return !kvm_ad_enabled();
/*
* Note, instruction fetches and writes are mutually exclusive, ignore
* the "exec" flag.
*/
return fault->write;
}
/*