KVM: x86/mmu: Reuse the current root if possible for fast switch (0be44352) · Commits · 戴 / test

Reuse the current root when possible instead of grabbing a different root from the array of cached roots. Doing so avoids unnecessary MMU switches and also fixes a quirk where KVM can't reuse roots without creating multiple roots since the cache is a victim cache, i.e. roots are added to the cache when they're "evicted", not when they are created. The quirk could be fixed by adding roots to the cache on creation, but that would reduce the effective size of the cache as one of its entries would be burned to track the current root. Reusing the current root is especially helpful for nested virt as the current root is almost always usable for the "new" MMU on nested VM-entry/VM-exit. Cc: Vitaly Kuznetsov <vkuznets@redhat.com> Signed-off-by:

Sean Christopherson <sean.j.christopherson@intel.com> Signed-off-by:

Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

+12 −3

Original line number	Diff line number	Diff line
		@@ -4253,6 +4253,14 @@ static void nonpaging_init_context(struct kvm_vcpu *vcpu,
		context->nx = false;
		}

		static inline bool is_root_usable(struct kvm_mmu_root_info *root, gpa_t cr3,
		union kvm_mmu_page_role role)
		{
		return (role.direct \|\| cr3 == root->cr3) &&
		VALID_PAGE(root->hpa) && page_header(root->hpa) &&
		role.word == page_header(root->hpa)->role.word;
		}

		/*
		* Find out if a previously cached root matching the new CR3/role is available.
		* The current root is also inserted into the cache.
		@@ -4271,12 +4279,13 @@ static bool cached_root_available(struct kvm_vcpu *vcpu, gpa_t new_cr3,
		root.cr3 = mmu->root_cr3;
		root.hpa = mmu->root_hpa;

		if (is_root_usable(&root, new_cr3, new_role))
		return true;

		for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++) {
		swap(root, mmu->prev_roots[i]);

		if ((new_role.direct \|\| new_cr3 == root.cr3) &&
		VALID_PAGE(root.hpa) && page_header(root.hpa) &&
		new_role.word == page_header(root.hpa)->role.word)
		if (is_root_usable(&root, new_cr3, new_role))
		break;
		}

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