Commit 5c64aba5 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

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: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20220423034752.1161007-6-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 54275f74

arch/x86/kvm/mmu/mmu.c

+12 −7
Original line number Original line Diff line number Diff line
@@ -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)

static bool page_fault_can_be_fast(struct kvm_page_fault *fault)

{

{

	/*

	/*

	 * Do not fix the mmio spte with invalid generation number which

	 * Page faults with reserved bits set, i.e. faults on MMIO SPTEs, only

	 * need to be updated by slow page fault path.

	 * 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)

	if (fault->rsvd)

		return false;

		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:

	 * #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

	 *    SPTE is MMU-writable (determined later), the fault can be fixed

	 *    by setting the Writable bit, which can be done out of mmu_lock.

	 *    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;

}

}





/*

/*