linux-kernel - Re: [PATCH v2 2/2] KVM: SEV: Configure "ALLOWED_SEV

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Message-ID: <ZsfKYHFkWA-Rh23C@google.com>
Date: Thu, 22 Aug 2024 16:31:44 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kim Phillips <kim.phillips@....com>
Cc: kvm@...r.kernel.org, linux-coco@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, x86@...nel.org, 
	Tom Lendacky <thomas.lendacky@....com>, Michael Roth <michael.roth@....com>, 
	Ashish Kalra <ashish.kalra@....com>, "Nikunj A . Dadhania" <nikunj@....com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 2/2] KVM: SEV: Configure "ALLOWED_SEV_FEATURES" VMCB Field

On Thu, Aug 22, 2024, Kim Phillips wrote:
> AMD EPYC 5th generation processors have introduced a feature that allows
> the hypervisor to control the SEV_FEATURES that are set for, or by, a
> guest [1].  ALLOWED_SEV_FEATURES can be used by the hypervisor to enforce
> that SEV-ES and SEV-SNP guests cannot enable features that the
> hypervisor does not want to be enabled.
> 
> When ALLOWED_SEV_FEATURES is enabled, a VMRUN will fail if any
> non-reserved bits are 1 in SEV_FEATURES but are 0 in
> ALLOWED_SEV_FEATURES.

This may need additional uAPI so that userspace can opt-in.  Dunno.  I hope guests
aren't abusing features, but IIUC, flipping this on has the potential to break
existing VMs, correct?