Re: [Qemu-devel] [PATCH 5/5] hw: arm_gic_kvm: Add KVM VGIC save/restore logic

[Peter Maydell]

On 21 September 2013 04:50, Christoffer Dall
<address@hidden> wrote:
> On Fri, Sep 06, 2013 at 04:13:32PM +0100, Peter Maydell wrote:
>> >      /* these registers are mainly used for save/restore of KVM state */
>> >      uint8_t  binary_point[2][NCPU]; /* [0]: group 0, [1]: group 1 */
>> > +    uint32_t active_prio[4][NCPU]; /* implementation defined layout */
>>
>> You can't make this impdef in QEMU's state, that would mean
>> we couldn't do migration between implementations which
>> use different layouts. We need to pick a standard ("whatever
>> the ARM v2 GIC implementation is" seems the obvious choice)
>> and make the kernel convert if it's on an implementation which
>> doesn't follow that version.

> Implementation defined as in implementation defined in the
> architecture.  I didn't think it would make sense to choose a format for
> an a15 implementation, for example, and then translate to that format
> for other cores using the ARM gic.  Wouldn't migration only be support
> for same qemu model to same qemu model, in which case the format of this
> register would always be the same, and the kernel must return a format
> corresponding to the target cpu.  Am I missing something here?

I know it's architecturally impdef, but there are a couple of issues
here:
 *) moving to the 'create me an irqchip' API is separating out the
"which GIC do I have?" and "which CPU do I have?" questions
somewhat, so it seems a shame to tangle them up again

 *) for getting TCG<->KVM and KVM-with-non-host-CPU cases
right we need to do translation anyway, or at least think about it.
So we need to at minimum specifically document what the format
is for the CPUs we care about. At that point we might as well have
a standard format. IIRC the GIC spec defines a "this is the sensible
format" anyway.

In practice, for the v7 and v8 CPUs we support, what format do
they use?


-- PMM