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

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Message-ID: <91f7f729-1f45-8081-4ac6-af0c82b712e4@redhat.com>
Date:   Tue, 9 Jan 2018 11:32:31 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Liran Alon <LIRAN.ALON@...CLE.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 08/01/2018 20:09, Liran Alon wrote:
>>
>> +static inline bool cpu_has_spec_ctrl(void)
>> +{
>> +    u32 eax, ebx, ecx, edx;
>> +    cpuid_count(7, 0, &eax, &ebx, &ecx, &edx);
>> +
>> +    return edx & bit(KVM_CPUID_BIT_SPEC_CTRL);
> 
> Why not just "return cpuid_edx(7) & bit(KVM_CPUID_BIT_SPEC_CTRL);"?
> This is also consistent with how cpu_has_ibpb_support() is written.

Leaf 7 explicitly requires you to clear ECX (there could be a leaf for
EAX=7,ECX=1 in the future).  Even though cpuid_edx does do that, it's
not clear from the function that cpuid_edx(7) would work.

Paolo