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

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Date:   Mon, 6 Mar 2023 13:16:25 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Takahiro Itazuri <itazur@...zon.com>, dave.hansen@...ux.intel.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, pbonzini@...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 Tue, Feb 28, 2023, Borislav Petkov wrote:
> On Tue, Feb 28, 2023 at 10:24:16PM +0000, Takahiro Itazuri wrote:
> > I'm still a kernel newbie and I don't have a strong opinion for that.
> > I just thought it would be helpful if the KVM_GET_SUPPORTED_CPUID API
> > returns the same security information as the host, as long as it is
> > harmless.
> 
> Not harmless - cpufeatures.h should contain flags which the kernel uses
> and not *every* CPUID bit out there.

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?