Re: [Xen-devel] [PATCH] xen: reuse the same pirq allocated when driver load first time

["Jan Beulich" <JBeulich@suse.com>]

>>> On 22.05.13 at 00:41, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> On Tue, May 21, 2013 at 10:50:09PM +0100, Stefano Stabellini wrote:
>> We have to be careful about this: the point of PHYSDEVOP_get_free_pirq is
>> that Linux can know for sure the pirq that is going to be used to map the
>> MSI by QEMU. If you modify is_free_pirq that way, suddenly the pirq
>> could be allocated for something else after Linux called
>> PHYSDEVOP_get_free_pirq and before QEMU called xc_physdev_map_pirq_msi.
> 
> Yes. And I think the 'is_free_pirq' modification above is incorrect.
> 
> I think the fix should be in the unmap_pirq code (hypervisor) to check
> if the arch.irq is either zero or PIRQ_ALLOCATED. Right now it only
> checks for zero.

Which check are you talking about? Looking at physdev_unmap_pirq()
I see none at all, unmap_domain_pirq() has a <= 0 check, and
unmap_domain_pirq_emuirq() again doesn't appear to have any.

If you're talking about unmap_domain_pirq(), then you'll need to
be careful: A negative value here doesn't necessarily mean
PIRQ_ALLOCATED, but could also come from another run that
found it necessary to force the unbind. Hence the definition of
PIRQ_ALLOCATED would then collide with the (unlikely?) case of
IRQ1 having got assigned to a guest. To be on the safe side, we
should therefore redefine PIRQ_ALLOCATED to say INT_MIN.

Jan