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

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Date:   Mon, 8 Mar 2021 12:50:07 +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 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()?

Thanks,
Yongji