linux-kernel - Re: [PATCH 1/2] KVM: x86: fix reporting of AMD speculation bug CPUID leaf

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Message-ID: <0e29f624-10f5-7ab5-1823-280f32732b68@redhat.com>
Date:   Mon, 19 Aug 2019 17:18:04 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: x86: fix reporting of AMD speculation bug CPUID
 leaf

On 16/08/19 23:45, Jim Mattson wrote:
> On Thu, Aug 15, 2019 at 12:41 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> The AMD_* bits have to be set from the vendor-independent
>> feature and bug flags, because KVM_GET_SUPPORTED_CPUID does not care
>> about the vendor and they should be set on Intel processors as well.
>> On top of this, SSBD, STIBP and AMD_SSB_NO bit were not set, and
>> VIRT_SSBD does not have to be added manually because it is a
>> cpufeature that comes directly from the host's CPUID bit.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> On AMD systems, aren't AMD_SSBD, AMD_STIBP, and AMD_SSB_NO set by
> inheritance from the host:
> 
> /* cpuid 0x80000008.ebx */
> const u32 kvm_cpuid_8000_0008_ebx_x86_features =
>         F(WBNOINVD) | F(AMD_IBPB) | F(AMD_IBRS) | F(AMD_SSBD) | F(VIRT_SSBD) |
>         F(AMD_SSB_NO) | F(AMD_STIBP) | F(AMD_STIBP_ALWAYS_ON);
> 
> I am curious why the cross-vendor settings go only one way. For
> example, you set AMD_STIBP on Intel processors that have STIBP, but
> you do not set INTEL_STIBP on AMD processors that have STIBP?
> Similarly, you set AMD_SSB_NO for Intel processors that are immune to
> SSB, but you do not set IA32_ARCH_CAPABILITIES.SSB_NO for AMD
> processors that are immune to SSB?
> 
> Perhaps there is another patch coming for reporting Intel bits on AMD?

I wasn't going to work on it but yes, they should be.  This patch just
fixed what was half-implemented.

Paolo