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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: 6b82ef2c9c ("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>
This commit is contained in:
Sean Christopherson 2021-01-13 12:50:30 -08:00 committed by Paolo Bonzini
parent 15e6a7e532
commit 8fc517267f
1 changed files with 1 additions and 1 deletions
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2
arch/x86/kvm/mmu/mmu.c
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@ -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.