Re: [PATCH] KVM: X86: MMU: no mmu_notifier_seq++ in kvm_age_hva

Peter Feiner
2016-09-27T15:31:57Z

On Tue, Sep 27, 2016 at 2:29 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> On 26/09/2016 19:45, Peter Feiner wrote:
>> The MMU notifier sequence number keeps GPA->HPA mappings in sync when
>> GPA->HPA lookups are done outside of the MMU lock (e.g., in
>> tdp_page_fault). Since kvm_age_hva doesn't change GPA->HPA, it's
>> unnecessary to increment the sequence number.
>>
>> Signed-off-by: Peter Feiner <pfeiner@google.com>
>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
>
> This needed some pondering, but it made sense in the end. :) Do you
> actually see an effect or is it just a cleanup?

Thanks Paolo. This is just a cleanup. I wanted to run it by you to
make sure *I* understood the role of the notifier sequence number.

I'm reworking this function in a fix for nested bug exposed by PML.
When PML is on, EPT A/D is enabled in the vmcs02 EPTP regardless of
the vmcs12's EPTP value. The problem is that enabling A/D changes the
behavior of L2's x86 page table walks as seen by L1. With A/D enabled,
x86 page table walks are always treated as EPT writes. The EPT test in
kvm-unit-tests's x86/vmx.c shows this failure.

> Related to this, what do you think of this patch?
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 3d4cc8cc56a3..23683d5f6640 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1617,16 +1617,8 @@ static int kvm_test_age_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
> struct rmap_iterator iter;
> int young = 0;
>
> - /*
> - * If there's no access bit in the secondary pte set by the
> - * hardware it's up to gup-fast/gup to set the access bit in
> - * the primary pte or in the page structure.
> - */
> - if (!shadow_accessed_mask)
> - goto out;
> -
> for_each_rmap_spte(rmap_head, &iter, sptep) {
> - if (*sptep & shadow_accessed_mask) {
> + if (!shadow_accessed_mask || (*sptep & shadow_accessed_mask)) {
> young = 1;
> break;
> }
>
> Since kvm_age_hva blows up "old" sPTEs, all PTEs that are there must be
> young...
>
> Paolo

Patch looks good. You can add "Reviewed-By: Peter Feiner <pfeiner@google.com>"

>> ---
>> arch/x86/kvm/mmu.c | 10 +---------
>> 1 file changed, 1 insertion(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 3d4cc8cc..dc6d1e8 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -1660,17 +1660,9 @@ int kvm_age_hva(struct kvm *kvm, unsigned long start, unsigned long end)
>> * This has some overhead, but not as much as the cost of swapping
>> * out actively used pages or breaking up actively used hugepages.
>> */
>> - if (!shadow_accessed_mask) {
>> - /*
>> - * We are holding the kvm->mmu_lock, and we are blowing up
>> - * shadow PTEs. MMU notifier consumers need to be kept at bay.
>> - * This is correct as long as we don't decouple the mmu_lock
>> - * protected regions (like invalidate_range_start|end does).
>> - */
>> - kvm->mmu_notifier_seq++;
>> + if (!shadow_accessed_mask)
>> return kvm_handle_hva_range(kvm, start, end, 0,
>> kvm_unmap_rmapp);
>> - }
>>
>> return kvm_handle_hva_range(kvm, start, end, 0, kvm_age_rmapp);
>> }
>>