Re: [PATCH for-4.6 3/4] xen/arm: vgic: notice if the vIRQ is not allocated when the guest enable it

[Julien Grall <julien.grall@linaro.org>]

Hi Ian,

On 13/01/15 15:55, Ian Campbell wrote:
> On Fri, 2014-12-12 at 14:43 +0000, Julien Grall wrote:
>> This help for guest interrupts debugging. If the vIRQ is not allocate,
>> this means that nothing is wired to it.
> 
> Should we short circuit the rest of the enable operation for this IRQ
> then? i.e. implement such writes as ignored, e.g. not reflect it in
> reads of ISENABLER etc.
> 
> What (if anything) does the GIC spec have to say on the subject?

"A register bit corresponding to an unimplemented interrupt is RAZ/WI."

The goal of this print was mostly for debugging physical IRQ routed to a
guest.

I could extend to ignore write to any register that should be RAZ/WI for
this specific interrupt.
But, I will have to think about possible race condition with the
hypercall to route a physical IRQ to the guest (see [1] and [2]).

The vIRQ is reserved before the physical IRQ is effectively routed. So
a guest may enable the vIRQ before this time lapse. Though, the patch
[2] protected for a such case.

Not sure if we should take care of a such case.

Regards,

-- 
Julien Grall