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

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Message-ID: <b3faa4a6-a65b-faf7-985a-b2771533c8bb@redhat.com>
Date:   Fri, 5 Mar 2021 11:42:31 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.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: [RFC v4 10/11] vduse: Introduce a workqueue for irq injection


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 :) ?

Thanks



>
> Thanks,
> Yongji
>