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

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Message-ID: <alpine.DEB.2.21.1802200937330.24268@nanos.tec.linutronix.de>
Date:   Tue, 20 Feb 2018 09:52:50 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        "valdis.kletnieks@...edu" <valdis.kletnieks@...edu>,
        Jon Masters <jcm@...masters.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs

On Mon, 19 Feb 2018, Linus Torvalds wrote:
> On Mon, Feb 19, 2018 at 4:13 PM, Alan Cox <gnomes@...rguk.ukuu.org.uk> wrote:
> >
> > In theory there's nothing stopping a guest getting a 'you are about to
> > gain/lose IBRS' message or having a new 'CPU' hotplugged and the old one
> > removed.
> 
> I'm not convinced we handle the case of hotplug CPU's with different
> CPU models correctly.
> 
> In fact, I'd be very surprised it it worked in the general case.

We pretend to work with different CPU models mostly by having per cpu
feature bits, but in practice this would fall apart in bits and pieces all
over the place. 

Thanks,

	tglx