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

Axel Rasmussen
2024-08-12T22:48:12Z

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.
>
> 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.

>
> Thanks,
>
> --
> Peter Xu
>