linux-kernel - Re: [PATCH 0/2] KVM: x86: Propagate AMD-specific IBRS bits to guests

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Date:   Mon, 6 Mar 2023 22:47:18 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Takahiro Itazuri <itazur@...zon.com>, dave.hansen@...ux.intel.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, tglx@...utronix.de, x86@...nel.org,
        zulinx86@...il.com
Subject: Re: [PATCH 0/2] KVM: x86: Propagate AMD-specific IBRS bits to guests

On 3/6/23 22:44, Borislav Petkov wrote:
>> I thought that the consensus was that adding unused-by-the-kernel flags to
>> cpufeatures.h is ok so long as the feature is hidden from /proc/cpuinfo and the
>> kernel already dedicates a word to the CPUID leaf?
> I guess we should finally write it down in Documentation/x86/cpuinfo.rst
> 
> And in case there's no dedicated word, it should be resorted to KVM-only
> feature flags.
> 
> In any case, I'd like for baremetal CPUID stuff to be decoupled from
> KVM's machinery as far as possible as both have different goals wrt
> feature flags.

It's very rare that KVM can provide a CPUID feature if the kernel has 
masked it, so if the kernel needs to know about a feature word than KVM 
most likely needs to know what kind of massaging the kernel has done.

Paolo