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Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
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From:   Julien Grall <julien@xen.org>
Message-ID: <28399fd1-9fe8-f31a-6ee8-e78de567155b@xen.org>
Date:   Mon, 8 Feb 2021 10:40:44 +0000
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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 )
{
   set IRQS_PENDING
   return;
}

do
{
   clear IRQS_PENDING
   handle_irq()
} while (IRQS_PENDING is set)

IRQ handling flow like handle_fasteoi_irq() looks like:

if ( irq in progress )
   return;

handle_irq()

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

Cheers,

-- 
Julien Grall