linux-kernel - Re: [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future CPUs

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Message-ID: <alpine.DEB.2.21.1802262054160.1402@nanos.tec.linutronix.de>
Date:   Mon, 26 Feb 2018 20:55:05 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Woodhouse <dwmw2@...radead.org>
cc:     karahmed@...zon.de, x86@...nel.org, kvm@...r.kernel.org,
        torvalds@...ux-foundation.org, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org, bp@...en8.de, peterz@...radead.org,
        jmattson@...gle.com, rkrcmar@...hat.com,
        arjan.van.de.ven@...el.com, dave.hansen@...el.com, mingo@...nel.org
Subject: Re: [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future
 CPUs

On Tue, 20 Feb 2018, David Woodhouse wrote:
> On Tue, 2018-02-20 at 11:42 +0100, Thomas Gleixner wrote:
> > 
> > > > However, Paolo is very insistent that taking the trap every time is
> > > > actually a lot *slower* than really frobbing IBRS on certain
> > > > microarchitectures, so my hand-waving "pfft, what did they expect?" is
> > > > not acceptable.
> > > > 
> > > > Which I think puts us back to the "throwing the toys out of the pram"
> > 
> > There are no more toys in the pram. I threw them all out weeks ago ...
> 
> One option is to take the patch as-is¹ with the trap on every access.
> As soon as Intel define that 'IBRS_ALL_AND_THE_BIT_IS_A_NOOP' bit in
> MSR_IA32_ARCH_CAPABILITIES, *then* we can expose it to guests directly
> again just as we do at the moment.

Arjan, is there any update on this?

Thanks,

	tglx