netdev - Re: Re: [RFC v4 10/11] vduse: Introduce a workqueue for irq injection

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Date:   Mon, 8 Mar 2021 15:16:43 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>, Bob Liu <bob.liu@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        Jens Axboe <axboe@...nel.dk>, bcrl@...ck.org,
        Jonathan Corbet <corbet@....net>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: Re: [RFC v4 10/11] vduse: Introduce a workqueue for irq injection

On Mon, Mar 8, 2021 at 3:02 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> On 2021/3/8 12:50 下午, Yongji Xie wrote:
> > On Mon, Mar 8, 2021 at 11:04 AM Jason Wang <jasowang@...hat.com> wrote:
> >>
> >> On 2021/3/5 4:12 下午, Yongji Xie wrote:
> >>> On Fri, Mar 5, 2021 at 3:37 PM Jason Wang <jasowang@...hat.com> wrote:
> >>>> On 2021/3/5 3:27 下午, Yongji Xie wrote:
> >>>>> On Fri, Mar 5, 2021 at 3:01 PM Jason Wang <jasowang@...hat.com> wrote:
> >>>>>> On 2021/3/5 2:36 下午, Yongji Xie wrote:
> >>>>>>> On Fri, Mar 5, 2021 at 11:42 AM Jason Wang <jasowang@...hat.com> wrote:
> >>>>>>>> On 2021/3/5 11:30 上午, Yongji Xie wrote:
> >>>>>>>>> On Fri, Mar 5, 2021 at 11:05 AM Jason Wang <jasowang@...hat.com> wrote:
> >>>>>>>>>> On 2021/3/4 4:58 下午, Yongji Xie wrote:
> >>>>>>>>>>> On Thu, Mar 4, 2021 at 2:59 PM Jason Wang <jasowang@...hat.com> wrote:
> >>>>>>>>>>>> On 2021/2/23 7:50 下午, Xie Yongji wrote:
> >>>>>>>>>>>>> This patch introduces a workqueue to support injecting
> >>>>>>>>>>>>> virtqueue's interrupt asynchronously. This is mainly
> >>>>>>>>>>>>> for performance considerations which makes sure the push()
> >>>>>>>>>>>>> and pop() for used vring can be asynchronous.
> >>>>>>>>>>>> Do you have pref numbers for this patch?
> >>>>>>>>>>>>
> >>>>>>>>>>> No, I can do some tests for it if needed.
> >>>>>>>>>>>
> >>>>>>>>>>> Another problem is the VIRTIO_RING_F_EVENT_IDX feature will be useless
> >>>>>>>>>>> if we call irq callback in ioctl context. Something like:
> >>>>>>>>>>>
> >>>>>>>>>>> virtqueue_push();
> >>>>>>>>>>> virtio_notify();
> >>>>>>>>>>>           ioctl()
> >>>>>>>>>>> -------------------------------------------------
> >>>>>>>>>>>               irq_cb()
> >>>>>>>>>>>                   virtqueue_get_buf()
> >>>>>>>>>>>
> >>>>>>>>>>> The used vring is always empty each time we call virtqueue_push() in
> >>>>>>>>>>> userspace. Not sure if it is what we expected.
> >>>>>>>>>> I'm not sure I get the issue.
> >>>>>>>>>>
> >>>>>>>>>> THe used ring should be filled by virtqueue_push() which is done by
> >>>>>>>>>> userspace before?
> >>>>>>>>>>
> >>>>>>>>> After userspace call virtqueue_push(), it always call virtio_notify()
> >>>>>>>>> immediately. In traditional VM (vhost-vdpa) cases, virtio_notify()
> >>>>>>>>> will inject an irq to VM and return, then vcpu thread will call
> >>>>>>>>> interrupt handler. But in container (virtio-vdpa) cases,
> >>>>>>>>> virtio_notify() will call interrupt handler directly. So it looks like
> >>>>>>>>> we have to optimize the virtio-vdpa cases. But one problem is we don't
> >>>>>>>>> know whether we are in the VM user case or container user case.
> >>>>>>>> Yes, but I still don't get why used ring is empty after the ioctl()?
> >>>>>>>> Used ring does not use bounce page so it should be visible to the kernel
> >>>>>>>> driver. What did I miss :) ?
> >>>>>>>>
> >>>>>>> Sorry, I'm not saying the kernel can't see the correct used vring. I
> >>>>>>> mean the kernel will consume the used vring in the ioctl context
> >>>>>>> directly in the virtio-vdpa case. In userspace's view, that means
> >>>>>>> virtqueue_push() is used vring's producer and virtio_notify() is used
> >>>>>>> vring's consumer. They will be called one by one in one thread rather
> >>>>>>> than different threads, which looks odd and has a bad effect on
> >>>>>>> performance.
> >>>>>> Yes, that's why we need a workqueue (WQ_UNBOUND you used). Or do you
> >>>>>> want to squash this patch into patch 8?
> >>>>>>
> >>>>>> So I think we can see obvious difference when virtio-vdpa is used.
> >>>>>>
> >>>>> But it looks like we don't need this workqueue in vhost-vdpa cases.
> >>>>> Any suggestions?
> >>>> I haven't had a deep thought. But I feel we can solve this by using the
> >>>> irq bypass manager (or something similar). Then we don't need it to be
> >>>> relayed via workqueue and vdpa. But I'm not sure how hard it will be.
> >>>>
> >>>    Or let vdpa bus drivers give us some information?
> >>
> >> This kind of 'type' is proposed in the early RFC of vDPA series. One
> >> issue is that at device level, we should not differ virtio from vhost,
> >> so if we introduce that, it might encourge people to design a device
> >> that is dedicated to vhost or virtio which might not be good.
> >>
> >> But we can re-visit this when necessary.
> >>
> > OK, I see. How about adding some information in ops.set_vq_cb()?
>
>
> I'm not sure I get this, maybe you can explain a little bit more?
>

For example, add an extra parameter for ops.set_vq_cb() to indicate
whether this callback will trigger the interrupt handler directly.

Thanks,
Yongji