Try to unprotect shadow pages if and only if indirect_shadow_pages is non- zero, i.e. iff there is at least one protected such shadow page. Pre- checking indirect_shadow_pages avoids taking mmu_lock for write when the gfn is write-protected by a third party, i.e. not for KVM shadow paging, and in the *extremely* unlikely case that a different task has already unprotected the last shadow page. Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- arch/x86/kvm/mmu/mmu.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index c34c8bbd61c8..dd62bd1e7657 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -2718,6 +2718,17 @@ bool kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa) gpa_t gpa = cr2_or_gpa; bool r; + /* + * Bail early if there aren't any write-protected shadow pages to avoid + * unnecessarily taking mmu_lock lock, e.g. if the gfn is write-tracked + * by a third party. Reading indirect_shadow_pages without holding + * mmu_lock is safe, as this is purely an optimization, i.e. a false + * positive is benign, and a false negative will simply result in KVM + * skipping the unprotect+retry path, which is also an optimization. + */ + if (!READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages)) + return false; + if (!vcpu->arch.mmu->root_role.direct) gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL); -- 2.46.0.469.g59c65b2a67-goog
