linux-kernel - [PATCH 4/4] KVM: X86: Don't unload MMU in kvm_vcpu_flush_tlb

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Date:   Tue, 19 Oct 2021 19:01:54 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 4/4] KVM: X86: Don't unload MMU in kvm_vcpu_flush_tlb_guest()

From: Lai Jiangshan <laijs@...ux.alibaba.com>

kvm_mmu_unload() destroys all the PGD caches.  Use the lighter
kvm_mmu_sync_roots() and kvm_mmu_sync_prev_roots() instead.

Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/kvm/mmu.h     |  1 +
 arch/x86/kvm/mmu/mmu.c | 16 ++++++++++++++++
 arch/x86/kvm/x86.c     | 11 +++++------
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 1ae70efedcf4..8e9dd63b68a9 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -79,6 +79,7 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
 int kvm_mmu_load(struct kvm_vcpu *vcpu);
 void kvm_mmu_unload(struct kvm_vcpu *vcpu);
 void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu);
+void kvm_mmu_sync_prev_roots(struct kvm_vcpu *vcpu);
 
 static inline int kvm_mmu_reload(struct kvm_vcpu *vcpu)
 {
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 900c7a157c99..fb45eeb8dd22 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3634,6 +3634,9 @@ static bool is_unsync_root(hpa_t root)
 {
 	struct kvm_mmu_page *sp;
 
+	if (!VALID_PAGE(root))
+		return false;
+
 	/*
 	 * Even if another CPU was marking the SP as unsync-ed simultaneously,
 	 * any guest page table changes are not guaranteed to be visible anyway
@@ -3706,6 +3709,19 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
 	write_unlock(&vcpu->kvm->mmu_lock);
 }
 
+void kvm_mmu_sync_prev_roots(struct kvm_vcpu *vcpu)
+{
+	unsigned long roots_to_free = 0;
+	int i;
+
+	for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
+		if (is_unsync_root(vcpu->arch.mmu->prev_roots[i].hpa))
+			roots_to_free |= KVM_MMU_ROOT_PREVIOUS(i);
+
+	/* sync prev_roots by simply freeing them */
+	kvm_mmu_free_roots(vcpu, vcpu->arch.mmu, roots_to_free);
+}
+
 static gpa_t nonpaging_gva_to_gpa(struct kvm_vcpu *vcpu, gpa_t vaddr,
 				  u32 access, struct x86_exception *exception)
 {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 13df3ca88e09..1771cd4bb449 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3251,15 +3251,14 @@ static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
 	++vcpu->stat.tlb_flush;
 
 	if (!tdp_enabled) {
-               /*
+		/*
 		 * A TLB flush on behalf of the guest is equivalent to
 		 * INVPCID(all), toggling CR4.PGE, etc., which requires
-		 * a forced sync of the shadow page tables.  Unload the
-		 * entire MMU here and the subsequent load will sync the
-		 * shadow page tables, and also flush the TLB.
+		 * a forced sync of the shadow page tables.  Ensure all the
+		 * roots are synced and the guest TLB in hardware is clean.
 		 */
-		kvm_mmu_unload(vcpu);
-		return;
+		kvm_mmu_sync_roots(vcpu);
+		kvm_mmu_sync_prev_roots(vcpu);
 	}
 
 	static_call(kvm_x86_tlb_flush_guest)(vcpu);
-- 
2.19.1.6.gb485710b