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

[Michael Roth]

On Thu, Feb 15, 2024 at 07:08:06PM +0100, Paolo Bonzini wrote:
> On Thu, Feb 15, 2024 at 6:55 PM Michael Roth <michael.roth@xxxxxxx> wrote:
> > > 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
> 
> Out of curiosity, where was this documented? While debug-swap was a
> pretty obvious culprit of the failed measurement, I didn't see any
> mention to it anywhere (and also didn't see any mention that old
> kernels would fail to boot in the KVM patches---which would have been
> a pretty clear indication that something like these patches was
> needed).

Yes, this was reactive rather than proactive guidance unfortunately,
resulting from various internal/external bug reports where we needed to
suggest the above-mentioned options.

In retrospect, I think we would've handled things differently as well.
Which is why I'm hoping it's possible to ease the pain of another
potential measurement change for those who've since incorporated
debug-swap into their measurement workflow. But maybe it's not
realistic...

> 
> > So those who'd taken approach a) would see another unexpected measurement
> > change when they eventually update to a newer kernel.
> 
> But they'd see it anyway if userspace starts disabling it by default.

My thinking was that this wording would be specific to KVM_SEV_INIT, as
opposed to KVM_SEV_INIT2 where disabling all features by default should
absolutely be the way to go.

But realistically, it's not easy for a user to tell whether their VMM is
using KVM_SEV_INIT vs KVM_SEV_INIT2, and it does seem possible that
having the defaults be different between the 2 would also cause some
pain down the road since even in looking at the documentation it
wouldn't be immediately clear whether or not debug-swap would be enabled.

So maybe your approach of always disabling by default and requiring
userspace to opt-in would be better in the long-run since this behavior
is fairly new from a distro perspective and it's likely only
developers/early adopters that we'd be sticking the genie back in the
bottle for.

-Mike

> In general, enabling _anything_ by default is a mistake in either KVM
> or userspace if you care about guest ABI (which you obviously do in
> the case of confidential computing).
> 
> Paolo
>