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

[Vishal Annapurve <vannapurve@google.com>]

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?

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/