Re: xen/evtchn and forced threaded irq

From: Juergen Gross
Date: Thu Feb 21 2019 - 03:17:16 EST


On 19/02/2019 18:31, Julien Grall wrote:
> Hi all,
>
> I have been looking at using Linux RT in Dom0. Once the guest is started,
> the console is ending to have a lot of warning (see trace below).
>
> After some investigation, this is because the irq handler will now be threaded.
> I can reproduce the same error with the vanilla Linux when passing the option
> 'threadirqs' on the command line (the trace below is from 5.0.0-rc7 that has
> not RT support).
>
> FWIW, the interrupt for port 6 is used to for the guest to communicate with
> xenstore.
>
> From my understanding, this is happening because the interrupt handler is now
> run in a thread. So we can have the following happening.
>
> Interrupt context | Interrupt thread
> |
> receive interrupt port 6 |
> clear the evtchn port |
> set IRQF_RUNTHREAD |
> kick interrupt thread |
> | clear IRQF_RUNTHREAD
> | call evtchn_interrupt
> receive interrupt port 6 |
> clear the evtchn port |
> set IRQF_RUNTHREAD |
> kick interrupt thread |
> | disable interrupt port 6
> | evtchn->enabled = false
> | [....]
> |
> | *** Handling the second interrupt ***
> | clear IRQF_RUNTHREAD
> | call evtchn_interrupt
> | WARN(...)
>
> I am not entirely sure how to fix this. I have two solutions in mind:
>
> 1) Prevent the interrupt handler to be threaded. We would also need to
> switch from spin_lock to raw_spin_lock as the former may sleep on RT-Linux.
>
> 2) Remove the warning

3) Split the handler (RT-only?) to a non-threaded part containing
everything until the "evtchn->enabled = false" and only then
kick the thread doing the rest, including the spin_lock().

Juergen