netdev - Re: Re: [RFC v3 08/11] vduse: Introduce VDUSE

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Date:   Thu, 28 Jan 2021 14:43:17 +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 v3 08/11] vduse: Introduce VDUSE - vDPA Device in Userspace

On Thu, Jan 28, 2021 at 2:14 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> On 2021/1/28 下午2:03, Yongji Xie wrote:
> >>>>> +
> >>>>> +static const struct file_operations vduse_domain_fops = {
> >>>>> +     .mmap = vduse_domain_mmap,
> >>>>> +     .release = vduse_domain_release,
> >>>>> +};
> >>>> It's better to explain the reason for introducing a dedicated file for
> >>>> mmap() here.
> >>>>
> >>> To make the implementation of iova_domain independent with vduse_dev.
> >> My understanding is that, the only usage for this is to:
> >>
> >> 1) support different type of iova mappings
> >> 2) or switch between iova domain mappings
> >>
> >> But I can't think of a need for this.
> >>
> > For example, share one iova_domain between several vduse devices.
>
>
> Interesting.
>
>
> >
> > And it will be helpful if we want to split this patch into iova domain
> > part and vduse device part. Because the page fault handler should be
> > paired with dma_map/dma_unmap.
>
>
> Ok.
>
> [...]
>
>
> >
> >>>> This looks not safe, let's use idr here.
> >>>>
> >>> Could you give more details? Looks like idr should not used in this
> >>> case which can not tolerate failure. And using a list to store the msg
> >>> is better than using idr when the msg needs to be re-inserted in some
> >>> cases.
> >> My understanding is the "unique" (probably need a better name) is a
> >> token that is used to uniquely identify a message. The reply from
> >> userspace is required to write with exact the same token(unique). IDR
> >> seems better but consider we can hardly hit 64bit overflow, atomic might
> >> be OK as well.
> >>
> >> Btw, under what case do we need to do "re-inserted"?
> >>
> > When userspace daemon receive the message but doesn't reply it before crash.
>
>
> Do we have code to do this?
>

Yes, in patch 9.

>
> >
> >>>> So we had multiple types of requests/responses, is this better to
> >>>> introduce a queue based admin interface other than ioctl?
> >>>>
> >>> Sorry, I didn't get your point. What do you mean by queue-based admin
> >>> interface? Virtqueue-based?
> >> Yes, a queue(virtqueue). The commands could be passed through the queue.
> >> (Just an idea, not sure it's worth)
> >>
> > I considered it before. But I found it still needs some extra works
> > (setup eventfd, set vring base and so on) to setup the admin virtqueue
> > before using it for communication. So I turn to use this simple way.
>
>
> Yes. We might consider it in the future.
>

Agree.

>
>
> >
> >>>> Any reason for such IOTLB invalidation here?
> >>>>
> >>> As I mentioned before, this is used to notify userspace to update the
> >>> IOTLB. Mainly for virtio-vdpa case.
> >> So the question is, usually, there could be several times of status
> >> setting during driver initialization. Do we really need to update IOTLB
> >> every time?
> >>
> > I think we can check whether there are some changes after the last
> > IOTLB updating here.
>
>
> So the question still, except reset (write 0), any other status that can
> affect IOTLB?
>

OK, I get your point. The status would not affect IOTLB. The reason
why we do IOTLB updating here is we can't do it in dma_map_ops which
might work in an atomic context. So I want to notify userspace to
update IOTLB before I/O is processed. Of course, it's not a must
because userspace can manually query it.

Thanks,
Yongji