linux-kernel - Re: [RFC PATCH v2 00/51] 1G page support for guest

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Message-ID: <aG14gLz9C_601TJ3@google.com>
Date: Tue, 8 Jul 2025 12:58:56 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Vishal Annapurve <vannapurve@...gle.com>
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Subject: Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

On Tue, Jul 08, 2025, Vishal Annapurve wrote:
> On Tue, Jul 8, 2025 at 11:03 AM Sean Christopherson <seanjc@...gle.com> wrote:
> Few points that seem important here:
> 1) Userspace can and should be able to only dictate if memory contents
> need to be preserved on shared to private conversion.

No, I was wrong, pKVM has use cases where it's desirable to preserve data on
private => shared conversions.

Side topic, if you're going to use fancy indentation, align the indentation so
it's actually readable.

>   -> For SNP/TDX VMs:
>        * Only usecase for preserving contents is initial memory
>          population, which can be achieved by:
>               -  Userspace converting the ranges to shared, populating the contents,
>                  converting them back to private and then calling SNP/TDX specific
>                  existing ABI functions.
>        * For runtime conversions, guest_memfd can't ensure memory contents are
>          preserved during shared to private conversions as the architectures
>          don't support that behavior.
>        * So IMO, this "preserve" flag doesn't make sense for SNP/TDX VMs, even

It makes sense, it's just not supported by the architecture *at runtime*.  Case
in point, *something* needs to allow preserving data prior to launching the VM.
If we want to go with the PRIVATE => SHARED => FILL => PRIVATE approach for TDX
and SNP, then we'll probably want to allow PRESERVE only until the VM image is
finalized.

>          if we add this flag, today guest_memfd should effectively mark this
>          unsupported based on the backing architecture support.
>
> 2) For pKVM, if userspace wants to specify a "preserve" flag then this

There is no "For pKVM".  We are defining uAPI for guest_memfd.  I.e. this statement
holds true for all implementations: PRESERVE is allowed based on the capabilities
of the architecture.

> So this topic is still orthogonal to "zeroing on private to shared conversion".

As above, no.  pKVM might not expose PRESERVE to _userspace_ since all current
conversions are initiated by the guest, but for guest_memfd itself, this is all
one and the same.