Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 08, 2025, Rick P Edgecombe wrote:
> 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.

Ya, by "vendor specific", I was also lumping in cases where the kernel would need
to zero memory in order to not end up with effectively undefined behavior.

> 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?

Because it's literally converting memory from shared to private?  And IICU, it's
not a one-time provisioning, e.g. memory can go:

  shared => fill => private => consume => shared => fill => private => consume

> Instead of a special provisioning operation like the others? (Xiaoyao's
> suggestion)

Are you referring to this suggestion?

 : And maybe a new flag for KVM_GMEM_CONVERT_PRIVATE for user space to
 : explicitly request that the page range is converted to private and the
 : content needs to be retained. So that TDX can identify which case needs
 : to call in-place TDH.PAGE.ADD.

If so, I agree with that idea, e.g. add a PRESERVE flag or whatever.  That way
userspace has explicit control over what happens to the data during conversion,
and KVM can reject unsupported conversions, e.g. PRESERVE is only allowed for
shared => private and only for select VM types.