Re: [PATCH 09/10] KVM: SEV: introduce KVM_SEV_INIT2 operation

[Michael Roth <michael.roth@amd.com>]

On Thu, Feb 15, 2024 at 06:28:18PM +0100, Paolo Bonzini wrote:
> On Thu, Feb 15, 2024 at 3:44 PM Michael Roth <michael.roth@amd.com> wrote:
> > What I mean is that if userspace is modified for these checks, it's
> > reasonable to also inform them that only VMSA features present in
> > those older kernels (i.e. debug-swap) will be available via KVM_SEV_INIT,
> > and for anything else they will need to use KVM_SEV_INIT.
> >
> > That way we can provide clear documentation on what to expect regarding
> > VMSA features for KVM_SEV_INIT and not have to have the "undefined"
> > wording: it'll never use anything other than debug-swap depending on the
> > module param setting.
> 
> Ah, I agree.
> 
> > That seems reasonable, but the main thing I was hoping to avoid was
> > another round of VMSA features changing out from underneath the covers
> > again. The module param setting is something we've needed to convey
> > internally/externally a good bit due to the fallout and making this
> > change would lead to another repeat. Not the end of the world but would
> > be nice to avoid if possible.
> 
> The fallout was caused by old kernels not supporting debug-swap and
> now by failing measurements. As far as I know there is no downside of
> leaving it disabled by default, and it will fix booting old guest
> kernels.

Yah, agreed on older guest kernels, but it's the measurement side of things
where we'd expect some additional fallout. The guidance was essentially that
if you run a newer host kernel with debug-swap support, you need either need
to:

  a) update your measurements to account for the additional VMSA feature
  b) disable debug-swap param to maintain previous behavior/measurement

So those who'd taken approach a) would see another unexpected measurement
change when they eventually update to a newer kernel.

-Mike

> 
> Paolo
>