linux-kernel - Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs

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Message-ID: <b55a1e8a-a70f-b007-e4d7-6864d7bdd683@redhat.com>
Date:   Tue, 13 Feb 2018 11:55:52 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>, tglx@...utronix.de,
        x86@...nel.org, kvm@...r.kernel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, arjan.van.de.ven@...el.com,
        dave.hansen@...el.com
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs

On 13/02/2018 11:53, David Woodhouse wrote:
>> 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.
> No, IBRS_ALL *does* make sense. It's not a complete fix, but it's as
> much of a fix as they should shoe-horn into the generation of CPUs
> which are currently going to the fabs.
> 
> With IBRS_ALL they presumably add tags to the predictions with the VMX
> mode and ring, to give complete protection against predictions being
> used in a more privileged mode. 

Yeah, but I still don't get why they need an MSR to turn those tags on.
Is it basically a chicken bit in the wrong direction?

Paolo