KVM: x86: Zap the oldest MMU pages, not the newest

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: 6b82ef2c ("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>

arch/x86/kvm/mmu/mmu.c

@@ -2417,7 +2417,7 @@ static unsigned long kvm_mmu_zap_oldest_mmu_pages(struct kvm *kvm,
 		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.