netdev - Re: [RFC v2 00/13] Introduce VDUSE

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Date:   Wed, 23 Dec 2020 18:59:41 +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>, sgarzare@...hat.com,
        Parav Pandit <parav@...dia.com>, akpm@...ux-foundation.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        axboe@...nel.dk, bcrl@...ck.org, corbet@....net,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC v2 00/13] Introduce VDUSE - vDPA Device in Userspace

On Wed, Dec 23, 2020 at 2:38 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> On 2020/12/22 下午10:52, Xie Yongji wrote:
> > This series introduces a framework, which can be used to implement
> > vDPA Devices in a userspace program. The work consist of two parts:
> > control path forwarding and data path offloading.
> >
> > In the control path, the VDUSE driver will make use of message
> > mechnism to forward the config operation from vdpa bus driver
> > to userspace. Userspace can use read()/write() to receive/reply
> > those control messages.
> >
> > In the data path, the core is mapping dma buffer into VDUSE
> > daemon's address space, which can be implemented in different ways
> > depending on the vdpa bus to which the vDPA device is attached.
> >
> > In virtio-vdpa case, we implements a MMU-based on-chip IOMMU driver with
> > bounce-buffering mechanism to achieve that.
>
>
> Rethink about the bounce buffer stuffs. I wonder instead of using kernel
> pages with mmap(), how about just use userspace pages like what vhost did?
>
> It means we need a worker to do bouncing but we don't need to care about
> annoying stuffs like page reclaiming?
>

Now the I/O bouncing is done in the streaming DMA mapping routines
which can be called from interrupt context. If we put this into a
kworker, that means we need to synchronize with a kworker in an
interrupt context. I think it can't work.

Thanks,
Yongji