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

Peter Xu
2024-08-12T18:58:13Z

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.

Thanks,

--
Peter Xu