Commit 9aa41879 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

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: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20201218003139.2167891-5-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent dde81f94

arch/x86/kvm/mmu/mmu.c

+13 −7
Original line number Diff line number Diff line
@@ -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;

}