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Message-ID: <ZR2EyUULbRpXW8wK@luigi.stachecki.net>
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
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