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Subjectxen/evtchn and forced threaded irq
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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

None of them are ideals. Do you have an opionion/better suggestion?

[ 127.192087] Interrupt for port 6, but apparently not enabled; per-user 0000000078d39c7f
[ 127.200333] WARNING: CPU: 0 PID: 2553 at drivers/xen/evtchn.c:167 evtchn_interrupt+0xfc/0x120
[ 127.208799] Modules linked in:
[ 127.211939] CPU: 0 PID: 2553 Comm: irq/52-evtchn:x Tainted: G W
5.0.0-rc7-00023-g2a3d41623699 #1257
[ 127.222374] Hardware name: ARM Juno development board (r2) (DT)
[ 127.228381] pstate: 40000005 (nZcv daif -PAN -UAO)
[ 127.233256] pc : evtchn_interrupt+0xfc/0x120
[ 127.237607] lr : evtchn_interrupt+0xfc/0x120
[ 127.241952] sp : ffff000012d2bd60
[ 127.245347] x29: ffff000012d2bd60 x28: ffff00001015d608
[ 127.250741] x27: ffff8008b39de400 x26: ffff00001015d330
[ 127.256137] x25: ffff00001015d2d4 x24: 0000000000000001
[ 127.261532] x23: ffff00001015d570 x22: 0000000000000034
[ 127.266926] x21: 0000000000000000 x20: ffff8008b7f02400
[ 127.272322] x19: ffff8008b3aba000 x18: 0000000000000037
[ 127.277717] x17: 0000000000000000 x16: 0000000000000000
[ 127.283112] x15: 00000000fffffff0 x14: 0000000000000000
[ 127.288507] x13: 3030303030207265 x12: 000000000000000c
[ 127.293902] x11: ffff000010ea0a88 x10: 0000000000000000
[ 127.299297] x9 : 00000000fffb9fff x8 : 0000000000000000
[ 127.304701] x7 : 0000000000000001 x6 : ffff8008bac5c240
[ 127.310087] x5 : ffff8008bac5c240 x4 : 0000000000000000
[ 127.315483] x3 : ffff8008bac64708 x2 : ffff8008b39de400
[ 127.320877] x1 : 893ac9b38837b800 x0 : 0000000000000000
[ 127.326272] Call trace:
[ 127.328802] evtchn_interrupt+0xfc/0x120
[ 127.332804] irq_forced_thread_fn+0x38/0x98
[ 127.337066] irq_thread+0x190/0x238
[ 127.340636] kthread+0x134/0x138
[ 127.343942] ret_from_fork+0x10/0x1c
[ 127.347593] ---[ end trace 1d3fa385877cc18b ]---


Cheers,

--
Julien Grall

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