[PATCH 5.10 12/23] KVM: x86: Zap the oldest MMU pages, not the newest

Greg Kroah-Hartman
2021-02-25T10:23:38Z

From: Sean Christopherson <seanjc@google.com>

commit 8fc517267fb28576dfca2380cc2497a2454b8fae upstream.

Walk the list of MMU pages in reverse in kvm_mmu_zap_oldest_mmu_pages().
The list is FIFO, meaning new pages are inserted at the head and thus
the oldest pages are at the tail. Using a "forward" iterator causes KVM
to zap MMU pages that were just added, which obliterates guest
performance once the max number of shadow MMU pages is reached.

Fixes: 6b82ef2c9cf1 ("KVM: x86/mmu: Batch zap MMU pages when recycling oldest pages")
Reported-by: Zdenek Kaspar <zkaspar82@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210113205030.3481307-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/mmu/mmu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2409,7 +2409,7 @@ static unsigned long kvm_mmu_zap_oldest_
return 0;

restart:
- list_for_each_entry_safe(sp, tmp, &kvm->arch.active_mmu_pages, link) {
+ list_for_each_entry_safe_reverse(sp, tmp, &kvm->arch.active_mmu_pages, link) {
/*
* Don't zap active root pages, the page itself can't be freed
* and zapping it will just force vCPUs to realloc and reload.