Re: [Xen-devel] [PATCH v2] libxl_qmp: wait for completion of device removal

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Wed, Jul 03, 2019 at 01:56:13PM +0800, Chao Gao wrote:
> To remove a device from a domain, a qmp command is sent to qemu. But it is
> handled by qemu asychronously. Even the qmp command is claimed to be done,
> the actual handling in qemu side may happen later.
> This behavior brings two questions:
> 1. Attaching a device back to a domain right after detaching the device from
> that domain would fail with error:
> 
> libxl: error: libxl_qmp.c:341:qmp_handle_error_response: Domain 1:received an
> error message from QMP server: Duplicate ID 'pci-pt-60_00.0' for device
> 
> 2. Accesses to PCI configuration space in Qemu may overlap with later device
> reset issued by 'xl' or by pciback.
> 
> In order to avoid mentioned questions, wait for the completion of device
> removal by querying all pci devices using qmp command and ensuring the target
> device isn't listed. Only retry 5 times to avoid 'xl' potentially being 
> blocked
> by qemu.
> 
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> ---
> Changes in v2:
>  - Break out early if we found an error during querying pci devices.
>  - Print a message to warn admin that device removal may not be done
>    in device model's side.

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

Thanks,

-- 
Anthony PERARD

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