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Date: Tue, 8 Aug 2023 11:08:09 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Christoph Hellwig <hch@...radead.org>,
virtualization@...ts.linux-foundation.org,
"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH vhost v11 05/10] virtio_ring: introduce virtqueue_dma_dev()
On Tue, Aug 8, 2023 at 10:52 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Tue, 8 Aug 2023 10:26:04 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Mon, Aug 7, 2023 at 2:15 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > >
> > > On Wed, 2 Aug 2023 09:49:31 +0800, Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > > On Tue, 1 Aug 2023 12:17:47 -0400, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > > > > On Fri, Jul 28, 2023 at 02:02:33PM +0800, Xuan Zhuo wrote:
> > > > > > On Tue, 25 Jul 2023 19:07:23 +0800, Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > > > > > On Tue, 25 Jul 2023 03:34:34 -0400, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > > > > > > > On Tue, Jul 25, 2023 at 10:13:48AM +0800, Xuan Zhuo wrote:
> > > > > > > > > On Mon, 24 Jul 2023 09:43:42 -0700, Christoph Hellwig <hch@...radead.org> wrote:
> > > > > > > > > > On Thu, Jul 20, 2023 at 01:21:07PM -0400, Michael S. Tsirkin wrote:
> > > > > > > > > > > Well I think we can add wrappers like virtio_dma_sync and so on.
> > > > > > > > > > > There are NOP for non-dma so passing the dma device is harmless.
> > > > > > > > > >
> > > > > > > > > > Yes, please.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > I am not sure I got this fully.
> > > > > > > > >
> > > > > > > > > Are you mean this:
> > > > > > > > > https://lore.kernel.org/all/20230214072704.126660-8-xuanzhuo@linux.alibaba.com/
> > > > > > > > > https://lore.kernel.org/all/20230214072704.126660-9-xuanzhuo@linux.alibaba.com/
> > > > > > > > >
> > > > > > > > > Then the driver must do dma operation(map and sync) by these virtio_dma_* APIs.
> > > > > > > > > No care the device is non-dma device or dma device.
> > > > > > > >
> > > > > > > > yes
> > > > > > > >
> > > > > > > > > Then the AF_XDP must use these virtio_dma_* APIs for virtio device.
> > > > > > > >
> > > > > > > > We'll worry about AF_XDP when the patch is posted.
> > > > > > >
> > > > > > > YES.
> > > > > > >
> > > > > > > We discussed it. They voted 'no'.
> > > > > > >
> > > > > > > http://lore.kernel.org/all/20230424082856.15c1e593@kernel.org
> > > > > >
> > > > > >
> > > > > > Hi guys, this topic is stuck again. How should I proceed with this work?
> > > > > >
> > > > > > Let me briefly summarize:
> > > > > > 1. The problem with adding virtio_dma_{map, sync} api is that, for AF_XDP and
> > > > > > the driver layer, we need to support these APIs. The current conclusion of
> > > > > > AF_XDP is no.
> > > > > >
> > > > > > 2. Set dma_set_mask_and_coherent, then we can use DMA API uniformly inside
> > > > > > driver. This idea seems to be inconsistent with the framework design of DMA. The
> > > > > > conclusion is no.
> > > > > >
> > > > > > 3. We noticed that if the virtio device supports VIRTIO_F_ACCESS_PLATFORM, it
> > > > > > uses DMA API. And this type of device is the future direction, so we only
> > > > > > support DMA premapped for this type of virtio device. The problem with this
> > > > > > solution is that virtqueue_dma_dev() only returns dev in some cases, because
> > > > > > VIRTIO_F_ACCESS_PLATFORM is supported in such cases.
> >
> > Could you explain the issue a little bit more?
> >
> > E.g if we limit AF_XDP to ACESS_PLATFROM only, why does
> > virtqueue_dma_dev() only return dev in some cases?
>
> The behavior of virtqueue_dma_dev() is not related to AF_XDP.
>
> The return value of virtqueue_dma_dev() is used for the DMA APIs. So it can
> return dma dev when the virtio is with ACCESS_PLATFORM. If virtio is without
> ACCESS_PLATFORM then it MUST return NULL.
>
> In the virtio-net driver, if the virtqueue_dma_dev() returns dma dev,
> we can enable AF_XDP. If not, we return error to AF_XDP.
Yes, as discussed, just having wrappers in the virtio_ring and doing
the switch there. Then can virtio-net use them without worrying about
DMA details?
Thanks
>
> Thanks
>
>
>
>
> >
> > Thanks
> >
> > >Otherwise NULL is returned.
> > > > > > This option is currently NO.
> > > > > >
> > > > > > So I'm wondering what should I do, from a DMA point of view, is there any
> > > > > > solution in case of using DMA API?
> > > > > >
> > > > > > Thank you
> > > > >
> > > > >
> > > > > I think it's ok at this point, Christoph just asked you
> > > > > to add wrappers for map/unmap for use in virtio code.
> > > > > Seems like a cosmetic change, shouldn't be hard.
> > > >
> > > > Yes, that is not hard, I has this code.
> > > >
> > > > But, you mean that the wrappers is just used for the virtio driver code?
> > > > And we also offer the API virtqueue_dma_dev() at the same time?
> > > > Then the driver will has two chooses to do DMA.
> > > >
> > > > Is that so?
> > >
> > > Ping.
> > >
> > > Thanks
> > >
> > > >
> > > >
> > > > > Otherwise I haven't seen significant comments.
> > > > >
> > > > >
> > > > > Christoph do I summarize what you are saying correctly?
> > > > > --
> > > > > MST
> > > > >
> > > >
> > >
> >
>
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