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Date: Tue, 8 Jul 2025 01:08:01 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

On Mon, 2025-07-07 at 17:14 -0700, Vishal Annapurve wrote:
> > 
> > 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.

For TDX if we don't zero on conversion from private->shared we will be dependent
on behavior of the CPU when reading memory with keyid 0, which was previously
encrypted and has some protection bits set. I don't *think* the behavior is
architectural. So it might be prudent to either make it so, or zero it in the
kernel in order to not make non-architectual behavior into userspace ABI.

Up the thread Vishal says we need to support operations that use in-place
conversion (overloaded term now I think, btw). Why exactly is pKVM using
private/shared conversion for this private data provisioning? Instead of a
special provisioning operation like the others? (Xiaoyao's suggestion)

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