linux-kernel - Re: ublk-qcow2: ublk-qcow2 is available

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Date:   Fri, 4 Nov 2022 14:55:55 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     Stefan Hajnoczi <stefanha@...il.com>,
        Yongji Xie <xieyongji@...edance.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Ming Lei <tom.leiming@...il.com>,
        Ziyang Zhang <ZiyangZhang@...ux.alibaba.com>,
        io-uring@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Denis V. Lunev" <den@...nvz.org>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
Subject: Re: ublk-qcow2: ublk-qcow2 is available

On Thu, Nov 3, 2022 at 3:13 AM Stefan Hajnoczi <stefanha@...hat.com> wrote:
>
> On Tue, Nov 01, 2022 at 10:36:29AM +0800, Jason Wang wrote:
> > On Tue, Oct 25, 2022 at 8:02 PM Stefan Hajnoczi <stefanha@...il.com> wrote:
> > >
> > > On Tue, 25 Oct 2022 at 04:17, Yongji Xie <xieyongji@...edance.com> wrote:
> > > >
> > > > On Fri, Oct 21, 2022 at 2:30 PM Jason Wang <jasowang@...hat.com> wrote:
> > > > >
> > > > >
> > > > > 在 2022/10/21 13:33, Yongji Xie 写道:
> > > > > > On Tue, Oct 18, 2022 at 10:54 PM Stefan Hajnoczi <stefanha@...il.com> wrote:
> > > > > >> On Tue, 18 Oct 2022 at 09:17, Yongji Xie <xieyongji@...edance.com> wrote:
> > > > > >>> On Tue, Oct 18, 2022 at 2:59 PM Ming Lei <tom.leiming@...il.com> wrote:
> > > > > >>>> On Mon, Oct 17, 2022 at 07:11:59PM +0800, Yongji Xie wrote:
> > > > > >>>>> On Fri, Oct 14, 2022 at 8:57 PM Ming Lei <tom.leiming@...il.com> wrote:
> > > > > >>>>>> On Thu, Oct 13, 2022 at 02:48:04PM +0800, Yongji Xie wrote:
> > > > > >>>>>>> On Wed, Oct 12, 2022 at 10:22 PM Stefan Hajnoczi <stefanha@...il.com> wrote:
> > > > > >>>>>>>> On Sat, 8 Oct 2022 at 04:43, Ziyang Zhang <ZiyangZhang@...ux.alibaba.com> wrote:
> > > > > >>>>>>>>> On 2022/10/5 12:18, Ming Lei wrote:
> > > > > >>>>>>>>>> On Tue, Oct 04, 2022 at 09:53:32AM -0400, Stefan Hajnoczi wrote:
> > > > > >>>>>>>>>>> On Tue, 4 Oct 2022 at 05:44, Ming Lei <tom.leiming@...il.com> wrote:
> > > > > >>>>>>>>>>>> On Mon, Oct 03, 2022 at 03:53:41PM -0400, Stefan Hajnoczi wrote:
> > > > > >>>>>>>>>>>>> On Fri, Sep 30, 2022 at 05:24:11PM +0800, Ming Lei wrote:
> > > There are ways to minimize that cost:
> > > 1. The driver only needs to fetch the device's sq index when it has
> > > run out of sq ring space.
> > > 2. The device can include sq index updates with completions. This is
> > > what NVMe does with the CQE SQ Head Pointer field, but the
> > > disadvantage is that the driver has no way of determining the sq index
> > > until a completion occurs.
> >
> > Probably, but as replied in another thread, based on the numbers
> > measured from the networking test, I think the current virtio layout
> > should be sufficient for block I/O but might not fit for cases like
> > NFV.
>
> I remember that the Linux virtio_net driver doesn't rely on vq spinlocks
> because CPU affinity and the NAPI architecture ensure that everything is
> CPU-local. There is no need to protect the freelist explicitly because
> the functions cannot race.
>
> Maybe virtio_blk can learn from virtio_net...

It only works for RX where add and get could be all done in NAPI. But
this is not the case for TX (and virtio-blk).

Actually, if the free_list is the one thing that needs to be
serialized, there's no need to use lock at all. We can try to switch
to use ptr_ring instead.

Thanks

>
> Stefan