netdev - Re: [PATCH net-next 0/5] virtio-net: sq support premapped mode

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Message-ID: <1706161325.4430635-1-xuanzhuo@linux.alibaba.com>
Date: Thu, 25 Jan 2024 13:42:05 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.com>
Cc: netdev@...r.kernel.org,
 "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>,
 Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 virtualization@...ts.linux.dev,
 bpf@...r.kernel.org
Subject: Re: [PATCH net-next 0/5] virtio-net: sq support premapped mode

On Thu, 25 Jan 2024 11:39:28 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Tue, Jan 16, 2024 at 3:59 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > This is the second part of virtio-net support AF_XDP zero copy.
> >
> > The whole patch set
> > http://lore.kernel.org/all/20231229073108.57778-1-xuanzhuo@linux.alibaba.com
> >
> > ## About the branch
> >
> > This patch set is pushed to the net-next branch, but some patches are about
> > virtio core. Because the entire patch set for virtio-net to support AF_XDP
> > should be pushed to net-next, I hope these patches will be merged into net-next
> > with the virtio core maintains's Acked-by.
> >
> > ============================================================================
> >
> > ## AF_XDP
> >
> > XDP socket(AF_XDP) is an excellent bypass kernel network framework. The zero
> > copy feature of xsk (XDP socket) needs to be supported by the driver. The
> > performance of zero copy is very good. mlx5 and intel ixgbe already support
> > this feature, This patch set allows virtio-net to support xsk's zerocopy xmit
> > feature.
> >
> > At present, we have completed some preparation:
> >
> > 1. vq-reset (virtio spec and kernel code)
> > 2. virtio-core premapped dma
> > 3. virtio-net xdp refactor
> >
> > So it is time for Virtio-Net to complete the support for the XDP Socket
> > Zerocopy.
> >
> > Virtio-net can not increase the queue num at will, so xsk shares the queue with
> > kernel.
> >
> > On the other hand, Virtio-Net does not support generate interrupt from driver
> > manually, so when we wakeup tx xmit, we used some tips. If the CPU run by TX
> > NAPI last time is other CPUs, use IPI to wake up NAPI on the remote CPU. If it
> > is also the local CPU, then we wake up napi directly.
> >
> > This patch set includes some refactor to the virtio-net to let that to support
> > AF_XDP.
> >
> > ## performance
> >
> > ENV: Qemu with vhost-user(polling mode).
> > Host CPU: Intel(R) Xeon(R) Platinum 8163 CPU @ 2.50GHz
> >
> > ### virtio PMD in guest with testpmd
> >
> > testpmd> show port stats all
> >
> >  ######################## NIC statistics for port 0 ########################
> >  RX-packets: 19531092064 RX-missed: 0     RX-bytes: 1093741155584
> >  RX-errors: 0
> >  RX-nombuf: 0
> >  TX-packets: 5959955552 TX-errors: 0     TX-bytes: 371030645664
> >
> >
> >  Throughput (since last show)
> >  Rx-pps:   8861574     Rx-bps:  3969985208
> >  Tx-pps:   8861493     Tx-bps:  3969962736
> >  ############################################################################
> >
> > ### AF_XDP PMD in guest with testpmd
> >
> > testpmd> show port stats all
> >
> >   ######################## NIC statistics for port 0  ########################
> >   RX-packets: 68152727   RX-missed: 0          RX-bytes:  3816552712
> >   RX-errors: 0
> >   RX-nombuf:  0
> >   TX-packets: 68114967   TX-errors: 33216      TX-bytes:  3814438152
> >
> >   Throughput (since last show)
> >   Rx-pps:      6333196          Rx-bps:   2837272088
> >   Tx-pps:      6333227          Tx-bps:   2837285936
> >   ############################################################################
> >
> > But AF_XDP consumes more CPU for tx and rx napi(100% and 86%).
> >
> > ## maintain
> >
> > I am currently a reviewer for virtio-net. I commit to maintain AF_XDP support in
> > virtio-net.
> >
> > Please review.
> >
>
> Rethink of the whole design, I have one question:
>
> The reason we need to store DMA information is to harden the virtqueue
> to make sure the DMA unmap is safe. This seems redundant when the
> buffer were premapped by the driver, for example:
>
> Receive queue maintains DMA information, so it doesn't need desc_extra to work.
>
> So can we simply
>
> 1) when premapping is enabled, store DMA information by driver itself

YES. this is simpler. And this is more convenience.
But the driver must allocate memory to store the dma info.

> 2) don't store DMA information in desc_extra

YES. But the desc_extra memory is wasted. The "next" item is used.
Do you think should we free the desc_extra when the vq is premapped mode?

Thanks.


>
> Would this be simpler?
>
> Thanks
>