linux-kernel - Re: [PATCH 1/2] KVM: SEV: IBPB-on-Entry guest support

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Message-ID: <2295adbc-835f-4a84-934b-b7aba65137a8@amd.com>
Date: Fri, 30 Jan 2026 08:56:07 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Borislav Petkov <bp@...en8.de>, Kim Phillips <kim.phillips@....com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 linux-coco@...ts.linux.dev, x86@...nel.org,
 Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>, K Prateek Nayak <kprateek.nayak@....com>,
 Nikunj A Dadhania <nikunj@....com>, Michael Roth <michael.roth@....com>,
 Naveen Rao <naveen.rao@....com>, David Kaplan <david.kaplan@....com>,
 stable@...nel.org
Subject: Re: [PATCH 1/2] KVM: SEV: IBPB-on-Entry guest support

On 1/30/26 06:32, Borislav Petkov wrote:
> On Thu, Jan 29, 2026 at 04:32:49PM -0600, Kim Phillips wrote:
>> Not *all* SNP features are implemented in all guest kernel versions, and,
>> well, for those that don't require explicit guest code support, perhaps it's
>> because they aren't necessarily well defined and validated in all hardware
>> versions...
> 
> Ok, can you add *this* feature to SNP_FEATURES_PRESENT? If not, why not?

It can be added. Any of the features added to SNP_FEATURES_PRESENT that
aren't set in the SNP_FEATURES_IMPL_REQ bitmap are really a no-op. The
SNP_FEATURES_PRESENT bitmap is meant to contain whatever bits are set in
SNP_FEATURES_IMPL_REQ when an implementation has been implemented for the
guest.

But, yeah, we could add all the bits that aren't set in
SNP_FEATURES_IMPL_REQ to SNP_FEATURES_PRESENT if it makes it clearer.

If we do that, it should probably be a separate patch (?) that also
rewords the comment above SNP_FEATURES_PRESENT

Thanks,
Tom

>