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SubjectRe: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts
On 2012-06-01 19:03, Alex Williamson wrote:
> On Fri, 2012-06-01 at 18:39 +0200, Jan Kiszka wrote:
>> On 2012-06-01 18:16, Alex Williamson wrote:
>>> The kernel no longer allows us to pass NULL for a hard interrupt
>>> handler without IRQF_ONESHOT. Should have been using this flag
>>> anyway.
>>
>> This make the IRQ handling tail a bit slower (due to
>> irq_finalize_oneshot). MSIs are edge-triggered, so there was no need for
>> masking in theory.
>
> Aren't these asynchronous since we can theoretically do
> irq_finalize_oneshot while the guest is servicing the device?

If it runs on a different CPU. But usually it's more efficient to have
handler and user on the same CPU. And this work has to be processed
somewhere.

>
>> Hmm, can't we trust the information that an IRQ
>> grabbed here is really a MSI type?
>
>
> Apparently not, comment added with this check (1c6c6952):
>
> * The interrupt was requested with handler = NULL, so
> * we use the default primary handler for it. But it
> * does not have the oneshot flag set. In combination
> * with level interrupts this is deadly, because the
> * default primary handler just wakes the thread, then
> * the irq lines is reenabled, but the device still
> * has the level irq asserted. Rinse and repeat....
> *
> * While this works for edge type interrupts, we play
> * it safe and reject unconditionally because we can't
> * say for sure which type this interrupt really
> * has. The type flags are unreliable as the
> * underlying chip implementation can override them.

I was talking about KVM here: Can't the KVM device assignment code
ensure that only MSIs are registered as such so that the above concerns
no longer apply?

Jan

--
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux


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