linux-kernel - Re: [PATCH 1/7] KVM: x86: add SPEC_CTRL and IBPB

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Date:   Tue, 9 Jan 2018 12:18:25 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     jmattson@...gle.com, aliguori@...zon.com, thomas.lendacky@....com,
        dwmw@...zon.co.uk, bp@...en8.de
Subject: Re: [PATCH 1/7] KVM: x86: add SPEC_CTRL and IBPB_SUPPORT accessors

On 09/01/2018 12:14, David Hildenbrand wrote:
>>  
>> +/* These are scattered features in cpufeatures.h. */
>> +#define KVM_CPUID_BIT_AVX512_4VNNIW	2
>> +#define KVM_CPUID_BIT_AVX512_4FMAPS	3
>> +#define KVM_CPUID_BIT_SPEC_CTRL		26
>> +#define KVM_CPUID_BIT_STIBP		27
> I can see that STIBP is never checked in KVM code but only forwarded to
> the guest if available.
> 
> I am wondering if we would have to check against that, too, before
> issuing a FEATURE_SET_IBPB.

STIBP is "single-threaded indirect branch prediction".  SPEC_CTRL always
implies that FEATURE_SET_IBPB is available.

Paolo