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Re: [Xen-devel] xen/evtchn and forced threaded irq

Hi Boris,

On 2/20/19 9:46 PM, Boris Ostrovsky wrote:
On 2/20/19 3:46 PM, Julien Grall wrote:
(+ Andrew and Jan for feedback on the event channel interrupt)

Hi Boris,

Thank you for the your feedback.

On 2/20/19 8:04 PM, Boris Ostrovsky wrote:
On 2/20/19 1:05 PM, Julien Grall wrote:

On 20/02/2019 17:07, Boris Ostrovsky wrote:
On 2/20/19 9:15 AM, Julien Grall wrote:
Hi Boris,

Thank you for your answer.

On 20/02/2019 00:02, Boris Ostrovsky wrote:
On Tue, Feb 19, 2019 at 05:31:10PM +0000, Julien Grall wrote:
Hi all,

I have been looking at using Linux RT in Dom0. Once the guest is
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

    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

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

2) Remove the warning

I think access to evtchn->enabled is racy so (with or without the
warning) we can't use it reliably.

Thinking about it, it would not be the only issue. The ring is sized
to contain only one instance of the same event. So if you receive
twice the event, you may overflow the ring.

Hm... That's another argument in favor of "unthreading" the handler.

I first thought it would be possible to unthread it. However,
wake_up_interruptible is using a spin_lock. On RT spin_lock can sleep,
so this cannot be used in an interrupt context.

So I think "unthreading" the handler is not an option here.

That sounds like a different problem. I.e. there are two issues:
* threaded interrupts don't work properly (races, ring overflow)
* evtchn_interrupt() (threaded or not) has spin_lock(), which is not
going to work for RT

I am afraid that's not correct, you can use spin_lock() in threaded
interrupt handler.

In non-RT handler -- yes, but not in an RT one (in fact, isn't this what
you yourself said above?)

In RT-linux, interrupt handlers are threaded by default. So the handler will not run in the interrupt context. Hence, it will be safe to call spin_lock.

However, if you force the handler to not be threaded (IRQF_NO_THREAD), it will run in interrupt context.

Another alternative could be to queue the irq if !evtchn->enabled
handle it in evtchn_write() (which is where irq is supposed to be
What do you mean by queue? Is it queueing in the ring?

No, I was thinking about having a new structure for deferred

Hmmm, I am not entirely sure what would be the structure here. Could
you expand your thinking?

Some sort of a FIFO that stores {irq, data} tuple. It could obviously be
implemented as a ring but not necessarily as Xen shared ring (if that's
what you were referring to).

The underlying question is what happen if you miss an interrupt. Is it
going to be ok?

This I am not sure about. I thought yes since we are signaling the
process only once.

I have CCed Andrew and Jan to see if they can help here.


Julien Grall

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