Re: [PATCH] virtio-blk: support polling I/O

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Mar 14, 2022 at 09:33:08PM +0900, Suwan Kim wrote:
> On Mon, Mar 14, 2022 at 02:14:53PM +0800, Jason Wang wrote:
> > 
> > 在 2022/3/11 下午11:28, Suwan Kim 写道:
> > > diff --git a/include/uapi/linux/virtio_blk.h b/include/uapi/linux/virtio_blk.h
> > > index d888f013d9ff..3fcaf937afe1 100644
> > > --- a/include/uapi/linux/virtio_blk.h
> > > +++ b/include/uapi/linux/virtio_blk.h
> > > @@ -119,8 +119,9 @@ struct virtio_blk_config {
> > >   	 * deallocation of one or more of the sectors.
> > >   	 */
> > >   	__u8 write_zeroes_may_unmap;
> > > +	__u8 unused1;
> > > -	__u8 unused1[3];
> > > +	__virtio16 num_poll_queues;
> > >   } __attribute__((packed));
> > 
> > 
> > This looks like a implementation specific (virtio-blk-pci) optimization, how
> > about other implementation like vhost-user-blk?
> 
> I didn’t consider vhost-user-blk yet. But does vhost-user-blk also
> use vritio_blk_config as kernel-qemu interface?
> 
> Does vhost-user-blk need additional modification to support polling
> in kernel side?

I think QEMU's --device vhost-user-blk-pci will work with this patch
series because QEMU passes the struct virtio_blk_config from the
vhost-user-blk server to the guest. If a new vhost-user-blk server
supports num_poll_queues then the guest will see the config field.

However, the new feature bit that was discussed in another sub-thread
needs to be added to hw/block/vhost-user-blk.c:user_feature_bits[] and
QEMU needs to be recompiled.

Stefan

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