linux-kernel - Re: [PATCH] x86: KVM: Add feature flag for AMD's FsGsKernelGsBaseNonSerializing

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Date:   Thu, 5 Oct 2023 09:22:44 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Dave Hansen <dave.hansen@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jiaxi Chen <jiaxi.chen@...ux.intel.com>,
        Kim Phillips <kim.phillips@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: KVM: Add feature flag for AMD's FsGsKernelGsBaseNonSerializing

On Wed, Oct 4, 2023 at 12:59 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, Oct 03, 2023 at 07:44:51PM -0700, Jim Mattson wrote:
> > The business of declaring breaking changes to the architectural
> > specification in a CPUID bit has never made much sense to me.
>
> How else should they be expressed then?
>
> In some flaky PDF which changes URLs whenever the new corporate CMS gets
> installed?
>
> Or we should do f/m/s matching which doesn't make any sense for VMs?
>
> When you think about it, CPUID is the best thing we have.

Every time a new defeature bit is introduced, it breaks existing
hypervisors, because no one can predict ahead of time that these bits
have to be passed through.

I wonder if we could convince x86 CPU vendors to put all defeature
bits under a single leaf, so that we can just set the entire leaf to
all 1's in KVM_GET_SUPPORTED_CPUID.