Re: The root cause of failure of access_tracking_perf_test in a nested guest

[Mingwei Zhang <mizhang@google.com>]

On Mon, Sep 26, 2022 at 1:50 AM Emanuele Giuseppe Esposito
<eesposit@redhat.com> wrote:
>
>
>
> Am 23/09/2022 um 22:28 schrieb David Matlack:
> > On Fri, Sep 23, 2022 at 12:25:00PM -0700, Jim Mattson wrote:
> >> On Fri, Sep 23, 2022 at 3:16 AM Maxim Levitsky <mlevitsk@redhat.com> wrote:
> >>>
> >>> Because of this, when the guest clears the accessed bit in its nested EPT entries, KVM doesn't
> >>> notice/intercept it and corresponding EPT sptes remain the same, thus later the guest access to
> >>> the memory is not intercepted and because of this doesn't turn back
> >>> the accessed bit in the guest EPT tables.
> >>
> >> Does the guest execute an INVEPT after clearing the accessed bit?
> >
> > No, that's the problem. In L1, access_tracking_perf_test is using
> > page_idle to mark guest memory as idle, which results in clear_young()
> > notifiers being sent to KVM clear access bits. clear_young() is
> > explicitly allowed to omit flushes, so KVM happily obliges.
> >
> >       /*
> >        * clear_young is a lightweight version of clear_flush_young. Like the
> >        * latter, it is supposed to test-and-clear the young/accessed bitflag
> >        * in the secondary pte, but it may omit flushing the secondary tlb.
> >        */
> >       int (*clear_young)(struct mmu_notifier *subscription,
> >                          struct mm_struct *mm,
> >                          unsigned long start,
> >                          unsigned long end);
> >
> > We could modify page_idle so that KVM performs TLB flushes. For example,
> > add a mechanism for userspace to trigger a TLB flush. Or change
> > page_idle to use clear_flush_young() (although that would be incredibly
> > expensive since page_idle only allows clearing one pfn at a time). But
> > I'm not sure creating a new userspace API just for this test is really
> > worth it, especially with multigen LRU coming soon.

Can we add an operation that causes KVM to flush guest TLB explicitly?
For instance, we can use any operation that causes a change in
EPT/NPT, which would invoke an explicit TLB flush.  E.g., enabling
dirty logging will do the job. Alternatively, adding a memslot for the
guest, letting the guest touch it and then removing it at host level
will also flush the TLB. I believe the both should be architecturally
neutral and the latter seems more stable.

In any case, would an explicit TLB suffice in this case? I think this
will cause the zapping of PTEs in L0 EPT/NPT.

Thanks.
-Mingwei