netdev - Re: [PATCH vhost v6 09/10] virtio_net: set premapped mode by find

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Date: Sun, 7 Apr 2024 12:24:00 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux.dev, "Michael S. Tsirkin" <mst@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH vhost v6 09/10] virtio_net: set premapped mode by find_vqs()

On Mon, Apr 1, 2024 at 11:10 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Mon, 1 Apr 2024 09:40:07 +0800, Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > On Fri, 29 Mar 2024 11:20:08 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > > On Thu, Mar 28, 2024 at 4:27 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > >
> > > > On Thu, 28 Mar 2024 16:05:02 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > > > > On Wed, Mar 27, 2024 at 7:14 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > > > >
> > > > > > Now, the virtio core can set the premapped mode by find_vqs().
> > > > > > If the premapped can be enabled, the dma array will not be
> > > > > > allocated. So virtio-net use the api of find_vqs to enable the
> > > > > > premapped.
> > > > > >
> > > > > > Judge the premapped mode by the vq->premapped instead of saving
> > > > > > local variable.
> > > > > >
> > > > > > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > > > > > ---
> > > > >
> > > > > I wonder what's the reason to keep a fallback when premapped is not enabled?
> > > >
> > > > Rethink this.
> > > >
> > > > I think you are right. We can remove the fallback.
> > > >
> > > > Because we have the virtio dma apis that wrap all the cases.
> > > > So I will remove the fallback from the virtio-net in next version.
> > >
> > > Ok.
> > >
> > > >
> > > > But we still need to export the premapped to the drivers.
> > > > Because we can enable the AF_XDP only when premapped is true.
> > >
> > > I may miss something but it should work like
> > >
> > > enable AF_XDP -> enable remapping
> > >
> > > So can we fail during remapping enablement?
> >
> >
> > YES.
> >
> > Enabling the premapped mode may fail, then we must stop to enable AF_XDP.
> >
> > AF-XDP requires that we export the dma dev to the af-xdp.
> > We can do that only when the virtio core works with use_dma_api.
> > Other other side, if we support the page-pool in future, we may have the
> > same requirement.
>
> Rethink this.
>
> Enable premapped MUST NOT fail. No care the use_dma_api is true or not, because
> we have the DMA APIs for virtio. Then the virtio-net rx will work with
> premapped (I will make the big mode work with premapped mode)

Just to make sure we're at the same page. Rx will always work in the
mode or pre mapping. So we can easily fail the probe if we fail to
enable RX premapping?

>
> AF_XDP checks the virtqueue_dma_dev() when enabling.
>
> But disabling premapped mode may fail, because that virtio ring need to
> allocate memory for dma.

That's kind of too tricky, what if we just allocate the memory for dma
unconditionally?

Thanks

>
> Thanks.
>
>
>
> >
> >
> > Thanks.
> >
> >
> > >
> > > THanks
> > >
> > > >
> > > > Thanks
> > > >
> > > >
> > > > >
> > > > > Thanks
> > > > >
> > > >
> > >
> >
>