linux-kernel - [PATCH v7 01/23] KVM: x86/mmu: Optimize MMU page cache lookup for all direct SPs

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Date:   Wed, 22 Jun 2022 15:26:48 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     maz@...nel.org, anup@...infault.org, seanjc@...gle.com,
        bgardon@...gle.com, peterx@...hat.com, maciej.szmigiero@...cle.com,
        kvmarm@...ts.cs.columbia.edu, linux-mips@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, pfeiner@...gle.com,
        jiangshanlai@...il.com, dmatlack@...gle.com
Subject: [PATCH v7 01/23] KVM: x86/mmu: Optimize MMU page cache lookup for all direct SPs

From: David Matlack <dmatlack@...gle.com>

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@...il.com>
Reviewed-by: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Peter Xu <peterx@...hat.com>
Signed-off-by: David Matlack <dmatlack@...gle.com>
Message-Id: <20220516232138.1783324-2-dmatlack@...gle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.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 27b2a5603496..c0afb4f1c8ae 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2000,7 +2000,6 @@ 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;
@@ -2060,7 +2059,8 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 			continue;
 		}
 
-		if (direct_mmu)
+		/* unsync and write-flooding only apply to indirect SPs. */
+		if (sp->role.direct)
 			goto trace_get_page;
 
 		if (sp->unsync) {
-- 
2.31.1