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

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Date:   Tue, 20 Feb 2018 14:08:26 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "valdis.kletnieks@...edu" <valdis.kletnieks@...edu>
CC:     Jon Masters <jcm@...masters.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.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

> >> For bonus points:  What should happen to a VM that is live migrated
> >> from one hypervisor to another, and the hypervisors have different
> >> IBRS support?
> >
> > Doctor Doctor it hurts when I do this....
> >
> > Migration tends to only work between HV's that are relatively
> > homogeneous, that's nothing new...
> 
> No Arjan, this is just wrong.  Well, I suppose it's right in the present
> tense with the IBRS mess on Skylake, but it's _not_ been true until last
> year.


I meant software wise. You're not going to live migrate from xen to kvm or backwards. or between very radically different versions of the kvm stack.