linux-kernel - Re: [PATCH v2 2/5] KVM: guest_memfd: Remove preparation tracking

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Message-ID: <CAGtprH_4K9TAH1DoeAW-UHTpMY2sNr8UUehoCPX_SKSQBNoERQ@mail.gmail.com>
Date: Mon, 15 Dec 2025 16:13:29 -0800
From: Vishal Annapurve <vannapurve@...gle.com>
To: Michael Roth <michael.roth@....com>
Cc: kvm@...r.kernel.org, linux-coco@...ts.linux.dev, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, thomas.lendacky@....com, pbonzini@...hat.com, 
	seanjc@...gle.com, vbabka@...e.cz, ashish.kalra@....com, 
	liam.merwick@...cle.com, david@...hat.com, ackerleytng@...gle.com, 
	aik@....com, ira.weiny@...el.com, yan.y.zhao@...el.com
Subject: Re: [PATCH v2 2/5] KVM: guest_memfd: Remove preparation tracking

On Mon, Dec 15, 2025 at 7:35 AM Michael Roth <michael.roth@....com> wrote:
>
> guest_memfd currently uses the folio uptodate flag to track:
>
>   1) whether or not a page has 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.
>
> Signed-off-by: Michael Roth <michael.roth@....com>

Reviewed-By: Vishal Annapurve <vannapurve@...gle.com>
Tested-By: Vishal Annapurve <vannapurve@...gle.com>