[PATCH 5.10.y] KVM: x86/mmu: Preserve nested TDP shadow page tables if they are used as roots

[Sean Christopherson <seanjc@google.com>]

From: Hyunwoo Kim <imv4bel@gmail.com>

[ Upstream commit 7a2c70e777a00c32ebafd376a6fe31ebb91c5b20 ]

kvm_mmu_zap_oldest_mmu_pages() excludes a shadow page whose root_count
is non-zero from top-level reclaim, because such a page cannot be
freed. The path in mmu_page_zap_pte() that recursively zaps a parentless
nested TDP child has no such check. As a result, a shadow page can
be zapped even if the page itself can't be freed; as the comment in
kvm_mmu_zap_oldest_mmu_pages() notes, zapping it will just force vCPUs
to rebuild the page.

As in top-level reclaim, do not recursively prepare zapping of a
nested TDP child whose root_count is non-zero.

Fixes: 2de4085cccea ("KVM: x86/MMU: Recursively zap nested TDP SPs when zapping last/only parent")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[sean: resolve minor conflict due to lack of lockless aging in 5.10]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 13cad4becfba..d2428c76d940 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2237,7 +2237,9 @@ static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
 			 * avoids retaining a large number of stale nested SPs.
 			 */
 			if (tdp_enabled && invalid_list &&
-			    child->role.guest_mode && !child->parent_ptes.val)
+			    child->role.guest_mode &&
+			    !child->root_count &&
+			    !child->parent_ptes.val)
 				return kvm_mmu_prepare_zap_page(kvm, child,
 								invalid_list);
 		}

base-commit: aca95a35f1bcc0626c4fa4d1252bf2d4c990a1a2
-- 
2.56.0.218.g1ec904b173-goog