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Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs
To:     David Woodhouse <dwmw2@infradead.org>, tglx@linutronix.de,
        x86@kernel.org, kvm@vger.kernel.org, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, arjan.van.de.ven@intel.com,
        dave.hansen@intel.com
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <02bd3fdd-1b73-6cab-fb09-38ba933396bd@redhat.com>
Date:   Tue, 13 Feb 2018 11:41:35 +0100
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On 13/02/2018 11:36, David Woodhouse wrote:
>>> - if the VM has IBRS_ALL, pass through the MSR when it is zero and
>>> intercept writes when it is one (no writes should happen)
>>>  
>>> - if the VM doesn't have IBRS_ALL, do as we are doing now, independent
>>> of what the host spectre_v2_ibrs_all() setting is.
>> We end up having to turn IBRS on again on vmexit then, taking care that
>> no conditional branch can go round it. So that becomes an
>> *unconditional* wrmsr or lfence in the vmexit path. We really don't
>> want that.
>
> Note that being able to keep it simple in KVM was basically what made
> the difference between me tolerating IBRS_ALL as Intel currently define
> it, and throwing my toys out of the pram (as I had done in the first
> iterations of this patch).

You have my vote. :)  Really, IBRS_ALL makes no sense and it would be
nice to know _why_ Intel is pushing something that makes no sense.

Paolo