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

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Date:   Tue, 20 Feb 2018 16:09:38 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.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

On 20/02/2018 15:59, Van De Ven, Arjan wrote:
> 
>>> 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

>From KVM's point of view it's mostly a matter of having a complete ioctl
API right.  I'm not that much worried by it.

Paolo