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Date:   Tue, 20 Feb 2018 01:03:19 +0000
From:   Alan Cox <gnomes@lxorguk.ukuu.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        "valdis.kletnieks@vt.edu" <valdis.kletnieks@vt.edu>,
        Jon Masters <jcm@jonmasters.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
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        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs
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On Mon, 19 Feb 2018 16:43:50 -0800
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, Feb 19, 2018 at 4:13 PM, Alan Cox <gnomes@lxorguk.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.

We don't. And even if we did the user space doesn't. It would be a
painful task to resolve.

Alan