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

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Date:   Tue, 20 Feb 2018 14:59:05 +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


> > 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.
> 
> Forwards migration to a radically newer version certainly happens.  So
> when the source hypervisor was too old to tell the VM about IBRS_ALL,
> for example, migration should work properly and the VM should perform
> well on the destination hypervisor.

that makes sense, compatibility in this direction can be done
and is useful as you move a fleet of servers forward

> 
> Backwards migration to older hypervisors also happens sometimes, but in
> general it creates more userspace than kernel issues.
> 

that direction is obviously harder