Re: [PATCH 10/19] KVM: Use follow_pfnmap API

Sean Christopherson
2024-08-12T23:44:42Z

On Mon, Aug 12, 2024, Axel Rasmussen wrote:
> On Mon, Aug 12, 2024 at 11:58 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Fri, Aug 09, 2024 at 10:23:20AM -0700, Axel Rasmussen wrote:
> > > On Fri, Aug 9, 2024 at 9:09 AM Peter Xu <peterx@redhat.com> wrote:
> > > >
> > > > Use the new pfnmap API to allow huge MMIO mappings for VMs. The rest work
> > > > is done perfectly on the other side (host_pfn_mapping_level()).
> > >
> > > I don't think it has to be done in this series, but a future
> > > optimization to consider is having follow_pfnmap just tell the caller
> > > about the mapping level directly. It already found this information as
> > > part of its walk. I think there's a possibility to simplify KVM /
> > > avoid it having to do its own walk again later.
> >
> > AFAIU pfnmap isn't special in this case, as we do the "walk pgtable twice"
> > idea also to a generic page here, so probably not directly relevant to this
> > patch alone.

Ya. My original hope was that KVM could simply walk the host page tables and get
whatever PFN+size it found, i.e. that KVM wouldn't care about pfn-mapped versus
regular pages. That might be feasible after dropping all of KVM's refcounting
shenanigans[*]? Not sure, haven't thought too much about it, precisely because
I too think it won't provide any meaningful performance boost.

> > But I agree with you, sounds like something we can consider trying. I
> > would be curious on whether the perf difference would be measurable in this
> > specific case, though. I mean, this first walk will heat up all the
> > things, so I'd expect the 2nd walk (which is lockless) later be pretty fast
> > normally.
>
> Agreed, the main benefit is probably just code simplification.

+1. I wouldn't spend much time, if any, trying to plumb the size back out.
Unless we can convert regular pages as well, it'd probably be more confusing to
have separate ways of getting the mapping size.