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

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Message-ID: <CACycT3u0+LTbtFMS75grKGZ2mnXzHnKug+HGWbf+nqVybqwkZQ@mail.gmail.com>
Date:   Fri, 5 Mar 2021 11:30:24 +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 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.

Thanks,
Yongji