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

[Suwan Kim <suwan.kim027@gmail.com>]

On Wed, Mar 16, 2022 at 10:02:13AM +0800, Jason Wang wrote:
> On Tue, Mar 15, 2022 at 10:43 PM Suwan Kim <suwan.kim027@gmail.com> wrote:
> >
> > On Tue, Mar 15, 2022 at 04:59:23PM +0800, Jason Wang wrote:
> > > On Mon, Mar 14, 2022 at 8:33 PM Suwan Kim <suwan.kim027@gmail.com> 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?
> > > >
> > >
> > > Yes, but see below.
> > >
> > >
> > > >
> > > > Does vhost-user-blk need additional modification to support polling
> > > > in kernel side?
> > > >
> > >
> > >
> > > No, but the issue is, things like polling looks not a good candidate for
> > > the attributes belonging to the device but the driver. So I have more
> > > questions:
> > >
> > > 1) what does it really mean for hardware virtio block devices?
> > > 2) Does driver polling help for the qemu implementation without polling?
> > > 3) Using blk_config means we can only get the benefit from the new device
> >
> > 1) what does it really mean for hardware virtio block devices?
> > 3) Using blk_config means we can only get the benefit from the new device
> >
> > This patch adds dedicated HW queue for polling purpose to virtio
> > block device.
> >
> > So I think it can be a new hw feature. And it can be a new device
> > that supports hw poll queue.
> 
> One possible issue is that the "poll" looks more like a
> software/driver concept other than the device/hardware.
> 
> >
> > BTW, I have other idea about it.
> >
> > How about adding “num-poll-queues" property as a driver parameter
> > like NVMe driver, not to QEMU virtio-blk-pci property?
> 
> It should be fine, but we need to listen to others.

To Michael, Stefan, Max

How about using driver parameter instead of virio_blk_config?

Regards,
Suwan Kim