Linux Virtualization - Aug 2023 - [PATCH vhost v11 05/10] virtio_ring: introduce virtqueue_dma_dev()

On Tue, 25 Jul 2023 19:07:23 +0800, Xuan Zhuo <xuanzhuo at
linux.alibaba.com> wrote:
> On Tue, 25 Jul 2023 03:34:34 -0400, "Michael S. Tsirkin" <mst
at redhat.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 at
infradead.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 at
linux.alibaba.com/
> > > https://lore.kernel.org/all/20230214072704.126660-9-xuanzhuo at
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 at 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. 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


>
> Thanks.
>
>
> >
> > --
> > MST
> >
>

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 at
linux.alibaba.com> wrote:
> > On Tue, 25 Jul 2023 03:34:34 -0400, "Michael S. Tsirkin"
<mst at redhat.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 at infradead.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
at linux.alibaba.com/
> > > > https://lore.kernel.org/all/20230214072704.126660-9-xuanzhuo
at 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 at 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. 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.
Otherwise I haven't seen significant comments.


Christoph do I summarize what you are saying correctly?
-- 
MST