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Date:   Thu, 1 Mar 2018 22:39:04 +0100
From:   Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        David Woodhouse <dwmw@amazon.co.uk>,
        KarimAllah Ahmed <karahmed@amazon.de>
Subject: Re: [PATCH] KVM: VMX: expose the host's ARCH_CAPABILITIES MSR to
 userspace
Message-ID: <20180301213205.GB29001@flask>
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 <20180226221300.GK22024@char.us.oracle.com>
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2018-02-26 17:13-0500, Konrad Rzeszutek Wilk:
> On Sat, Feb 24, 2018 at 01:52:26AM +0100, Paolo Bonzini wrote:
> > Use the new MSR feature framework to expose the ARCH_CAPABILITIES MSR to
> > userspace.  This way, userspace can access the capabilities even if it
> > does not have the permissions to read MSRs.
> 
> ... That is good but could you expand a bit of why it would want this?
> 
> I am 99% sure it is due to the lovely spectre_v2 mitigation but
> could you include that in the commit message so that in say a year
> folks would know what this is?

Userspace can currently get the MSR by creating a VCPU and reading its
MSR_IA32_ARCH_CAPABILITIES, because it is set from the hardware MSR.

I thought that "permissions to read MSRs" talked about hardware MSRs, so
the purpose of this patch would be a better interface, but I don't see
how if we keep the auto-setting on VCPU creation.

Is this aimed towards userspaces that want nothing else from KVM than
the MSR value?

Thanks.