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From: Paolo Bonzini <pbonzini@redhat.com>
Date: Thu, 15 Feb 2024 19:08:06 +0100
Message-ID: <CABgObfa_ktGybPcai=OgBbYMMvm4jS_Hehc-cdLdFoev68z-GQ@mail.gmail.com>
Subject: Re: [PATCH 09/10] KVM: SEV: introduce KVM_SEV_INIT2 operation
To: Michael Roth <michael.roth@amd.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, seanjc@google.com, 
	aik@amd.com, isaku.yamahata@intel.com, thomas.lendacky@amd.com
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On Thu, Feb 15, 2024 at 6:55=E2=80=AFPM Michael Roth <michael.roth@amd.com>=
 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 thin=
gs
> where we'd expect some additional fallout. The guidance was essentially t=
hat
> if you run a newer host kernel with debug-swap support, you need either n=
eed
> 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).

> 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.
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