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Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids

Hi Juergen,

On 08/02/2021 12:31, Jürgen Groß wrote:
On 08.02.21 13:16, Julien Grall wrote:

On 08/02/2021 12:14, Jürgen Groß wrote:
On 08.02.21 11:40, Julien Grall wrote:
Hi Juergen,

On 08/02/2021 10:22, Jürgen Groß wrote:
On 08.02.21 10:54, Julien Grall wrote:
... I don't really see how the difference matter here. The idea is to re-use what's already existing rather than trying to re-invent the wheel with an extra lock (or whatever we can come up).

The difference is that the race is occurring _before_ any IRQ is
involved. So I don't see how modification of IRQ handling would help.

Roughly our current IRQ handling flow (handle_eoi_irq()) looks like:

if ( irq in progress )

} while (IRQS_PENDING is set)

IRQ handling flow like handle_fasteoi_irq() looks like:

if ( irq in progress )


The latter flow would catch "spurious" interrupt and ignore them. So it would handle nicely the race when changing the event affinity.

Sure? Isn't "irq in progress" being reset way before our "lateeoi" is
issued, thus having the same problem again?

Sorry I can't parse this.

handle_fasteoi_irq() will do nothing "if ( irq in progress )". When is
this condition being reset again in order to be able to process another
It is reset after the handler has been called. See handle_irq_event().

I believe this will be the case before our "lateeoi" handling is
becoming active (more precise: when our IRQ handler is returning to
handle_fasteoi_irq()), resulting in the possibility of the same race we
are experiencing now.

I am a bit confused what you mean by "lateeoi" handling is becoming active. Can you clarify?

Note that are are other IRQ flows existing. We should have a look at them before trying to fix thing ourself.

Although, the other issue I can see so far is handle_irq_for_port() will update info->{eoi_cpu, irq_epoch, eoi_time} without any locking. But it is not clear this is what you mean by "becoming active".


Julien Grall



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