Re: [PATCH RFC v1 1/5] KVM: guest_memfd: Remove preparation tracking

Michael Roth
2025-07-15T22:56:26Z

On Tue, Jul 15, 2025 at 05:47:31AM -0700, Vishal Annapurve wrote:
> On Thu, Jun 12, 2025 at 5:55 PM Michael Roth <michael.roth@amd.com> wrote:
> >
> > guest_memfd currently uses the folio uptodate flag to track:
> >
> > 1) whether or not a page had been cleared before initial usage
> > 2) whether or not the architecture hooks have been issued to put the
> > page in a private state as defined by the architecture
> >
> > In practice, 2) is only actually being tracked for SEV-SNP VMs, and
> > there do not seem to be any plans/reasons that would suggest this will
> > change in the future, so this additional tracking/complexity is not
> > really providing any general benefit to guest_memfd users. Future plans
> > around in-place conversion and hugepage support, where the per-folio
> > uptodate flag is planned to be used purely to track the initial clearing
> > of folios, whereas conversion operations could trigger multiple
> > transitions between 'prepared' and 'unprepared' and thus need separate
> > tracking, will make the burden of tracking this information within
> > guest_memfd even more complex, since preparation generally happens
> > during fault time, on the "read-side" of any global locks that might
> > protect state tracked by guest_memfd, and so may require more complex
> > locking schemes to allow for concurrent handling of page faults for
> > multiple vCPUs where the "preparedness" state tracked by guest_memfd
> > might need to be updated as part of handling the fault.
> >
> > Instead of keeping this current/future complexity within guest_memfd for
> > what is essentially just SEV-SNP, just drop the tracking for 2) and have
> > the arch-specific preparation hooks get triggered unconditionally on
> > every fault so the arch-specific hooks can check the preparation state
> > directly and decide whether or not a folio still needs additional
> > preparation. In the case of SEV-SNP, the preparation state is already
> > checked again via the preparation hooks to avoid double-preparation, so
> > nothing extra needs to be done to update the handling of things there.
> >
>
> I believe this patch doesn't need to depend on stage1/stage2 and can
> be sent directly for review on kvm tip, is that right?

Yes, this was actually tested initially against kvm/next and should not
cause issues. I wanted to post the change in the context of in-place
conversion/hugetlb work to help motivate why we're considering the
change, but ideally we'd get this one applied soon-ish since the question
of "how to track preparation state" seems to be throwing a wrench into all
the planning activities and at the end of the day only SNP is making use
of it so it seems to be becoming more trouble than it's worth at a
fairly fast pace.

-Mike

>
> This update paired with zeroing modifications[1] will make uptodate
> flag redundant for guest_memfd memory.
>
> [1] https://lore.kernel.org/lkml/CAGtprH-+gPN8J_RaEit=M_ErHWTmFHeCipC6viT6PHhG3ELg6A@mail.gmail.com/