Re: [PATCH v2] libxl: Retry QMP PCI device_add

[Anthony PERARD <anthony.perard@citrix.com>]

On Fri, Apr 22, 2022 at 10:07:03AM -0400, Jason Andryuk wrote:
> PCI device assignment to an HVM with stubdom is potentially racy.  First
> the PCI device is assigned to the stubdom via the PV PCI protocol.  Then
> QEMU is sent a QMP command to attach the PCI device to QEMU running
> within the stubdom.  However, the sysfs entries within the stubdom may
> not have appeared by the time QEMU receives the device_add command
> resulting in errors like:
> 
> libxl_qmp.c:1838:qmp_ev_parse_error_messages:Domain 10:Could not open '/sys/bus/pci/devices/0000:00:1f.3/config': No such file or directory
> 
> This patch retries the device assignment up to 10 times with a 1 second
> delay between.  That roughly matches the overall hotplug timeout for
> pci_add_timeout.  pci_add_timeout's initialization is moved to
> do_pci_add since retries call into pci_add_qmp_device_add again.
> 
> The qmp_ev_parse_error_messages error is still printed since it happens
> at a lower level than the pci code controlling the retries.  With that,
> the "Retrying PCI add %d" message is also printed at ERROR level to
> clarify what is happening.
> 
> Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
> ---
> v2:
> Only retry when a stubdom is present.
> Move pci_add_timeout initialization.
> Use pas->aodev->ao directly.

Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

Thanks,

-- 
Anthony PERARD