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

Hi Dario,

On 22/09/16 09:43, Dario Faggioli wrote:
On Wed, 2016-09-21 at 20:28 +0100, Julien Grall wrote:
On 21/09/2016 16:45, Dario Faggioli wrote:
This does not seem to match with what has been said at some point
this thread... And if it's like that, how's that possible, if the
pcpus' ISAs are (even only slightly) different?

Right, at some point I mentioned that the set of errata and features
will be different between processor.

Yes, I read that, but wasn't (and still am not) sure about whether or
not that meant a vcpu can move freely between classes or not, in the
way that the scheduler does that.

In fact, you say:

With a bit of work in Xen, it would be possible to do move the vCPU
between big and LITTLE cpus. As mentioned above, we could sanitize
features to only enable a common set.
You can view the big.LITTLE
problem as a local live migration between two kind of CPUs.

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 don't need to recreate the vCPU every time you move from one set of CPUs to another one. Sorry for the confusion.

Julien Grall

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