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

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Message-ID: <CACGkMEug-=C+VQhkMYSgUKMC==04m7-uem_yC21bgGkKZh845w@mail.gmail.com>
Date: Thu, 25 Jan 2024 14:14:58 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.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, Jan 25, 2024 at 1:52 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Thu, 25 Jan 2024 13:42:05 +0800, Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > 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.

Right, and this looks like the common practice for most of the NIC drivers.

> >
> > > 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?
>
>
> struct vring_desc_extra {
>         dma_addr_t addr;                /* Descriptor DMA addr. */
>         u32 len;                        /* Descriptor length. */
>         u16 flags;                      /* Descriptor flags. */
>         u16 next;                       /* The next desc state in a list. */
> };
>
>
> The flags and the next are used whatever premapped or not.
>
> So I think we can add a new array to store the addr and len.

Yes.

> If the vq is premappd, the memory can be freed.

Then we need to make sure the premapped is set before find_vqs() etc.

>
> struct vring_desc_extra {
>         u16 flags;                      /* Descriptor flags. */
>         u16 next;                       /* The next desc state in a list. */
> };
>
> struct vring_desc_dma {
>         dma_addr_t addr;                /* Descriptor DMA addr. */
>         u32 len;                        /* Descriptor length. */
> };
>
> Thanks.

Thanks

>
> >
> > Thanks.
> >
> >
> > >
> > > Would this be simpler?
> > >
> > > Thanks
> > >
>