Re: [PATCH v2] libxl: wait for QEMU exit before restarting a domain

[Jiaqing Zhao <Zhao.Jiaqing@amd.com>]

On 2026-11-10 18:22, Jan Beulich wrote:
> On 10.11.2026 11:02, Jiaqing Zhao wrote:
>> The PID-based device model destruction path returns immediately after
>> sending SIGHUP, before QEMU has necessarily exited. When an HVM guest
>> reboots with VNC enabled, the new QEMU process may fail to bind the VNC
>> port because the old process still holds it.
>>
>> Wait for QEMU to exit in a child process after signaling it for at most
>> 10 seconds. If waiting fails or times out, log the error and retain the
>> existing domain destruction behaviour.
>>
>> Signed-off-by: Jiaqing Zhao <Zhao.Jiaqing@amd.com>
>> ---
>> v2:
>> * Share the fork, error handling and child exit handling between UID and
>>   PID-based destruction paths.
>> * Reduce the device-model stop timeout from 60 to 10 seconds.
> 
> As per my v1 comments I continue to question this.

I looked further into QEMU's shutdown path. After receiving SIGHUP,
QEMU calls bdrv_drain_all() to wait for outstanding block I/O requests
and bdrv_flush_all() to flush its block devices. These could take
longer than 10 seconds on slow storage.

While this does not identify the timeout you observed with SR-IOV,
my earlier measurements are insufficient to prove that 10s is enough.
I'll restore the 60s timeout in v3.

>> * Handle unexpected errors from kill(pid, 0) explicitly.
>> * Fix misspelled "signaling" in commit message.
>> * Coding style fixes against tools/libs/light/CODING_STYLE.
>> ---
>> Cc: Anthony PERARD <anthony.perard@vates.tech>
>> Cc: Juergen Gross <jgross@suse.com>
>> Cc: Roger Pau Monné <roger@xenproject.com>
> 
> You will want to use a correct address here (.org rather than .com).
> 
> Jan

That was my mistake, sorry.

By the way, on MAINTAINERS:390, Roger's address uses .com, maybe it need to
be fixed?