Re: [RFC] KVM: x86/mmu: Prefetch forward run of pages on TDP page faults

[James Houghton]

On Tue, Aug 25, 2026 at 7:27 AM Marco Marangoni <mamarang@xxxxxxxxxx> wrote:
>
> The TDP MMU installs one SPTE per fault, so faulting in guest memory via
> userfaultfd (e.g. snapshot restore) without hugetlbfs costs a VM-exit per
> 4KiB page. Mirror the shadow MMU's prefetch: after a 4KiB fault, resolve
> the forward run of host-present pages in the faulting leaf table (one
> guest 2MiB region) with one non-blocking GUP and fill the empty SPTEs.
> When userfaultfd populates in large chunks this maps up to 511 neighbours
> per fault, cutting EPT violations up to 512x.
>
> Touching 128MiB backed by userfaultfd with 2MiB UFFD_COPY chunks:
> c8i.metal-48xl 129.1 -> 100.3 ms (-22%), nested 495.8 -> 121.5 ms (-75%).
> With no batching (one copy per fault) there is a ~2-4% regression.

Hi Marco!

I think part of the problem in this case is that UFFDIO_COPY will
install 4K pages (IIRC), I think a more natural way to fix this
problem is to:

1. MADV_COLLAPSE after doing UFFDIO_COPY.
2. Make UFFDIO_COPY install PMDs when it is able to do so.

These don't solve the exact same problem, but really userfaultfd
should already try to install PMDs when it can (#2). If we have #2, #1
is mostly a no-op.

What do you think?