Subject: [PATCH] arm64/kvm: Add generic v8 KVM target

From: [email-masked] (Marc Zyngier)
Date: Fri, 03 Jul 2015 11:10:09 +0100

On 03/07/15 10:34, Peter Maydell wrote:
> On 3 July 2015 at 09:28, Marc Zyngier <[email-masked]> wrote:
>> On 03/07/15 09:12, Peter Maydell wrote:
>>> I would still like to see the proponents of this patch say
>>> what their model is for userspace support of cross-host migration,
>>> if we're abandoning the model the current API envisages.
>>
>> I thought we had discussed this above, and don't really see this as a
>> departure from the current model:
>>
>> - "-cpu host" results in "GENERIC" being used: VM can only be migrated
>> to the exact same HW (no cross-host migration). MIDR should probably
>> become RO.
>> - "-cpu host" results in "A57" (for example): VM can be migrated to a
>> variety of A57 platforms, and allow for some fuzzing on the revision (or
>> accept any revision).
>> - "-cpu a57" forces an A57 model to be emulated, always. It is always
>> possible to migrate such a VM on any host.
>>
>> I think only the first point is new, but the last two are what we have
>> (or what we should have).
> 
> Right, but the implicit idea of this GENERIC patch seems to
> be that new host CPU types don't get their own KVM_ARM_TARGET_*
> constant, and are thus forever unable to do cross-host migration.
> It's not clear to me why we'd want to have new CPUs be second
> class citizens like that.

I certainly don't want to see *any* CPU be a second class citizen. But
let's face it, we're adding more and more targets that don't implement
anything new, and just satisfy themselves with the generic implementation.

I see it as an incentive to provide something useful (tables of all the
registers with default values?) so that cross-host migration becomes a
reality instead of the figment of our imagination (as it is now). If it
wasn't already ABI, I'd have removed the existing targets until we have
something meaningful to put there.

Now, I also have my own doubts about cross-host migration (timers
anyone?). But I don't see the above as a change in policy. More as a way
to outline the fact that we currently don't have the right level of
information/infrastructure to support it at all.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...