linux-kernel - Re: [PATCH 09/10] KVM: SEV: introduce KVM_SEV

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Date: Thu, 15 Feb 2024 18:28:18 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Michael Roth <michael.roth@....com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, seanjc@...gle.com, 
	aik@....com, isaku.yamahata@...el.com, thomas.lendacky@....com
Subject: Re: [PATCH 09/10] KVM: SEV: introduce KVM_SEV_INIT2 operation

On Thu, Feb 15, 2024 at 3:44 PM Michael Roth <michael.roth@....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.

Paolo