Re: [PATCH v2 0/6] KVM: x86/mmu: Fast page fault support for the TDP MMU

[David Matlack <dmatlack@google.com>]

On Thu, Jul 01, 2021 at 07:00:51PM +0200, David Hildenbrand wrote:
> On 30.06.21 23:47, David Matlack wrote:
> > This patch series adds support for the TDP MMU in the fast_page_fault
> > path, which enables certain write-protection and access tracking faults
> > to be handled without taking the KVM MMU lock. This series brings the
> > performance of these faults up to par with the legacy MMU.
> > 
> > Since there is not currently any KVM test coverage for access tracking
> > faults, this series introduces a new KVM selftest,
> > access_tracking_perf_test. Note that this test relies on page_idle to
> > enable access tracking from userspace (since it is the only available
> > usersapce API to do so) and page_idle is being considered for removal
> > from Linux
> > (https://lore.kernel.org/linux-mm/20210612000714.775825-1-willy@infradead.org/).
> 
> Well, at least a new selftest that implicitly tests a part of page_idle --
> nice :)
> 
> Haven't looked into the details, but if you can live with page tables
> starting unpopulated and only monitoring what gets populated on r/w access,
> you might be able to achieve something similar using /proc/self/pagemap and
> softdirty handling.
> 
> Unpopulated page (e.g., via MADV_DISCARD) -> trigger read or write access ->
> sense if page populated in pagemap
> Populated page-> clear all softdirty bits -> trigger write access -> sense
> if page is softdirty in pagemap

Thanks for the suggestion. I modified by test to write 4 to
/proc/self/clear_refs rather than marking pages in page_idle. However,
by doing so I was no longer able to exercise KVM's fast_page_fault
handler [1].

It looks like the reason why is that clear_refs issues the
invalidate_range mmu notifiers, which will cause KVM to fully refault
the page from the host MM upon subsequent guest memory accesses.  In
contrast, page_idle uses clear_young which KVM can handle with
fast_page_fault.

Let me know if I misunderstood your suggestion though.

[1] https://www.kernel.org/doc/html/latest/virt/kvm/locking.html#exception

> 
> See https://lkml.kernel.org/r/20210419135443.12822-6-david@redhat.com for an
> example.
> 
> But I'm actually fairly happy to see page_idel getting used. Maybe you could
> extend that test using pagemap, if it's applicable to your test setup.
> 
> -- 
> Thanks,
> 
> David / dhildenb
>