linux-kernel - Re: [PATCH 0/5] KVM: x86: Fix breakage in KVM_SET

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Date:   Wed, 4 Oct 2023 11:29:13 -0400
From:   Tyler Stachecki <stachecki.tyler@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Leonardo Bras <leobras@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 0/5] KVM: x86: Fix breakage in KVM_SET_XSAVE's ABI

On Wed, Oct 04, 2023 at 07:51:17AM -0700, Sean Christopherson wrote:
> KVM's ABI has no formal notion of guest boot=>shutdown or live migration.  The
> myriad KVM_GET_* APIs allow taking a snapshot of guest state, and the KVM_SET_*
> APIs allow loading a snapshot of guest state.  Live migration is probably the most
> common use of those APIs, but there are other use cases.

I think the lightbulb just clicked, it is really this:

> No, the VM will not have less features, because KVM_SET_XSAVE loads *data*, not
> features [...]

I think I'm conflating the data vs. features aspect here and will have to
revisit my understanding of the code...

> > Ultimately, this problem is not really fixable if said features cannot be
> > removed.

> It's not about removing features.  The change you're asking for is to have KVM
> *silently* drop data.  Aside from the fact that such a change would break KVM's
> ABI, silently ignoring data that userspace has explicitly requested be loaded for
> a vCPU is incredibly dangerous.

Sorry if it came off that way - I fully understand and am resigned to the "you
break it, you keep both halves" nature of what I had initially proposed and
that it is not a generally tractable solution.

That being said, I genuinely appreciate your jump to action on this problem!

Thanks,
Tyler