Re: [Qemu-devel] [PATCH v2 3/6] hw/char: pl011 don't keep setting the IRQ if nothing changed

[Alex Bennée]

Peter Maydell <address@hidden> writes:

> On 12 March 2015 at 15:51, Peter Maydell <address@hidden> wrote:
>> On 4 March 2015 at 14:35, Alex Bennée <address@hidden> wrote:
>>> While observing KVM traces I can see additional IRQ calls on pretty much
>>> every MMIO access which is just plain inefficient. Only update the QEMU
>>> IRQ level if something has actually changed from last time. Otherwise we
>>> may be papering over other failure modes.
>
>> Consider this sequence of events:
>
> Incidentally, the code review rule of thumb that led me to
> construct that scenario is:
>  * state inside the device struct which doesn't correspond
>    to real hardware state is suspicious
>  * state which doesn't have any handling on migration
>    save/restore is doubly suspicious
> ...and then it was just a matter of "find the situation
> where this is broken" :-)
>
> If we want to avoid making syscalls back into KVM it might
> be better to attack the problem in the GIC object rather
> than in all the devices that might be connected to it.
> In general QEMU devices tend to assume they can just
> always call qemu_set_irq() and it's the other end's
> job to avoid doing anything too expensive in that situation.

Fair enough - I'll drop this patch on the re-spin

>
> -- PMM

-- 
Alex Bennée