Re: [Qemu-devel] [PATCH 3/3] virtio-scsi-dataplane: Use main thread BH to set BDS' aio context

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/02/2015 02:21, Fam Zheng wrote:
>> Could you just do set_aio_context for all devices when starting
>> dataplane?  For example with a new scsi_device_set_aio_context function
>> (and a new method in SCSIDeviceClass).
>>
>> Maybe I'm missing the obvious. :)
> 
> Per VQ IOThread stills needs it, in the case that guest switches the VQ of a
> disk on the fly.

Per VQ iothread is far away though.  Multiqueue aims at parallelizing
accesses *to the same disk from different CPUs*, not at parallelizing
accesses to different disks.  As long as bdrv_set_aio_context does
bdrv_drain_all, something has to change for multiqueue dataplane: either
stop using bdrv_set_aio_context, or stop doing bdrv_drain_all.

In either case, the changes are large enough that we shouldn't code for
a case that doesn't exist yet.

In addition, making the code more similar for virtio-blk and virtio-scsi
dataplane is good, because the same changes can apply to both in the future.

Paolo