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

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Date:   Fri, 16 Feb 2018 10:21:57 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>, 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
Cc:     Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs



On Fri, 2018-02-16 at 11:08 +0100, Paolo Bonzini wrote:
> On 16/02/2018 10:58, David Woodhouse wrote:
> > 
> > On Tue, 2018-02-13 at 11:41 +0100, Paolo Bonzini wrote:
> > 
> > > 
> > > 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. :)
> > 
> > I was taking that as assent to the patch... could I trouble you for an
> > explicit ack, please?
>
> No, it's a vote for throwing the toys out of the pram (or running away
> with the ball, if you prefer).
> 
> Unfortunately, if you want to have a higher-performance mode for
> IBRS_ALL that avoids rdmsr on vmexit, you have to do it as sketched above.


Why? With IBRS_ALL the guest *never* gets to affect the actual hardware
MSR, which is always on. The MSR is purely an emulated no-op. Why does
that affect migration?

Even if the guest doesn't have/support IBRS_ALL, and is frobbing the
(now emulated) MSR on every kernel entry/exit, that's *still* going to
be a metric shitload faster than what it *thought* it was doing.

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