linux-kernel - [PATCH v2 19/20] kvm: x86/mmu: Don't clear write flooding count for direct roots

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Date:   Wed, 14 Oct 2020 11:26:59 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Cannon Matthews <cannonmatthews@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: [PATCH v2 19/20] kvm: x86/mmu: Don't clear write flooding count for
 direct roots

Direct roots don't have a write flooding count because the guest can't
affect that paging structure. Thus there's no need to clear the write
flooding count on a fast CR3 switch for direct roots.

Tested by running kvm-unit-tests and KVM selftests on an Intel Haswell
machine. This series introduced no new failures.

This series can be viewed in Gerrit at:
	https://linux-review.googlesource.com/c/virt/kvm/kvm/+/2538

Signed-off-by: Ben Gardon <bgardon@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 2e8bf8d19c35a..3935c10278736 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4266,7 +4266,13 @@ static void __kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd,
 	 */
 	vcpu_clear_mmio_info(vcpu, MMIO_GVA_ANY);
 
-	__clear_sp_write_flooding_count(to_shadow_page(vcpu->arch.mmu->root_hpa));
+	/*
+	 * If this is a direct root page, it doesn't have a write flooding
+	 * count. Otherwise, clear the write flooding count.
+	 */
+	if (!new_role.direct)
+		__clear_sp_write_flooding_count(
+				to_shadow_page(vcpu->arch.mmu->root_hpa));
 }
 
 void kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd, bool skip_tlb_flush,
-- 
2.28.0.1011.ga647a8990f-goog