Re: [Xen-devel] [RFC 0/5] xen/arm: support big.little SoC

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Thu, 2016-09-22 at 12:24 +0100, Julien Grall wrote:
> On 22/09/16 09:43, Dario Faggioli wrote:
> > Local migration basically --from the vcpu perspective-- means
> > create a
> > new vcpu, stop the original vcpu, copy the state from original to
> > new,
> > destroy the original vcpu and start the new one. My point is that
> > this
> > is not something that can be done within nor initiated by the
> > scheduler, e.g., during a context switch or a vcpu wakeup!
> 
> By local migration, I meant from the perspective of the hypervisor.
> In 
> the hypervisor you have to trap feature registers and other 
> implementation defined registers to show the same value across all
> the 
> physical CPUs.
> 
You mean we trap feature registers during the (normal) execution of a
vcpu, because we want Xen to vet what's returned to the guest itself.
And that migration support, and hence the possibility that the guest
have been migrated to a cpu different than the one where it was
created, is already one of the reasons why this is necessary... right?

If yes, and if that's "all" we need, I think it should be fine.

> You don't need to recreate the vCPU every time you move from one set
> of 
> CPUs to another one. Sorry for the confusion.
> 
No, I am sorry... it's not you that you're making confusion, it's
probably me knowing to few about ARM, and did not think at the above
when you said "migration". :-)

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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