Message-ID: <CAGtprH_57HN4Psxr5MzAZ6k+mLEON2jVzrLH4Tk+Ws29JJuL4Q@mail.gmail.com>
Date: Mon, 7 Jul 2025 17:14:17 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
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Subject: Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

On Mon, Jul 7, 2025 at 4:25 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Jul 01, 2025, Vishal Annapurve wrote:
> > I would be curious to understand if we need zeroing on conversion for
> > Confidential VMs. If not, then the simple rule of zeroing on
> > allocation only will work for all usecases.
>
> Unless I'm misunderstanding what your asking, pKVM very specific does NOT want
> zeroing on conversion, because one of its use cases is in-place conversion, e.g.
> to fill a shared buffer and then convert it to private so that the buffer can be
> processed in the TEE.

Yeah, that makes sense. So "just zero on allocation" (and no more
zeroing during conversion) policy will work for pKVM.

>
> Some architectures, e.g. SNP and TDX, may effectively require zeroing on conversion,
> but that's essentially a property of the architecture, i.e. an arch/vendor specific
> detail.

Conversion operation is a unique capability supported by guest_memfd
files so my intention of bringing up zeroing was to better understand
the need and clarify the role of guest_memfd in handling zeroing
during conversion.

Not sure if I am misinterpreting you, but treating "zeroing during
conversion" as the responsibility of arch/vendor specific
implementation outside of guest_memfd sounds good to me.

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