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

[Fam Zheng <famz@redhat.com>]

On Fri, 02/13 10:38, Paolo Bonzini wrote:
> 
> 
> 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.

I think we should avoid duplicate everything on both virtio-blk and
virtio-scsi, so they will have differences.

Why do you think Per VQ iothread is far away? Limiting to 1 thread for the
whole scsi bus doesn't sound ultimate solution for me. I think it's not harder
than the MMIO safety work we have, and also somehow independent to it.

But yes, stop using bdrv_set_aio_context will be the other way to make it
right, just harder to do.

Fam