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Date:   Thu, 5 Oct 2023 09:35:39 -0700
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Subject: Re: [PATCH] x86: KVM: Add feature flag for AMD's
 FsGsKernelGsBaseNonSerializing
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To:     Jim Mattson <jmattson@google.com>, Borislav Petkov <bp@alien8.de>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Jiaxi Chen <jiaxi.chen@linux.intel.com>,
        Kim Phillips <kim.phillips@amd.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
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From:   Dave Hansen <dave.hansen@intel.com>
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On 10/5/23 09:22, Jim Mattson wrote:
> On Wed, Oct 4, 2023 at 12:59 AM Borislav Petkov <bp@alien8.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.

I hope I'm not throwing stones from a glass house here...

But I'm struggling to think of cases where Intel has read-only
"defeature bits" like this one.  There are certainly things like
MSR_IA32_MISC_ENABLE_FAST_STRING that can be toggled, but read-only
indicators of a departure from established architecture seems ...
suboptimal.

It's arguable that TDX changed a bunch of architecture like causing
exceptions on CPUID and MSRs that never caused exceptions before and
_that_ constitutes a defeature.  But that's the least of the problems
for a TDX VM. :)

(Seriously, I'm not trying to shame Intel's x86 fellow travelers here,
 just trying to make sure I'm not missing something).