[PATCH v6 01/22] KVM: x86/mmu: Optimize MMU page cache lookup for all direct SPs

David Matlack dmatlack at google.com
Mon May 16 16:21:17 PDT 2022

Commit fb58a9c345f6 ("KVM: x86/mmu: Optimize MMU page cache lookup for
fully direct MMUs") skipped the unsync checks and write flood clearing
for full direct MMUs. We can extend this further to skip the checks for
all direct shadow pages. Direct shadow pages in indirect MMUs (i.e.
shadow paging) are used when shadowing a guest huge page with smaller
pages. Such direct shadow pages, like their counterparts in fully direct
MMUs, are never marked unsynced or have a non-zero write-flooding count.

Checking sp->role.direct also generates better code than checking
direct_map because, due to register pressure, direct_map has to get
shoved onto the stack and then pulled back off.

No functional change intended.

Reviewed-by: Lai Jiangshan <jiangshanlai at gmail.com>
Reviewed-by: Sean Christopherson <seanjc at google.com>
Reviewed-by: Peter Xu <peterx at redhat.com>
Signed-off-by: David Matlack <dmatlack at google.com>
 arch/x86/kvm/mmu/mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index efe5a3dca1e0..774810d8a2ed 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2026,7 +2026,6 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 					     int direct,
 					     unsigned int access)
-	bool direct_mmu = vcpu->arch.mmu->root_role.direct;
 	union kvm_mmu_page_role role;
 	struct hlist_head *sp_list;
 	unsigned quadrant;
@@ -2070,7 +2069,8 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
-		if (direct_mmu)
+		/* unsync and write-flooding only apply to indirect SPs. */
+		if (sp->role.direct)
 			goto trace_get_page;
 		if (sp->unsync) {

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