Date:   Sat, 4 Feb 2017 11:10:36 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>, bjorn.topel@...il.com,
        ast@...com, alexander.duyck@...il.com, brouer@...hat.com
Cc:     john.r.fastabend@...el.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] af_packet: direct dma for packet ineterface



On 2017年01月28日 05:33, John Fastabend wrote:
> This adds ndo ops for upper layer objects to request direct DMA from
> the network interface into memory "slots". The slots must be DMA'able
> memory given by a page/offset/size vector in a packet_ring_buffer
> structure.
>
> The PF_PACKET socket interface can use these ndo_ops to do zerocopy
> RX from the network device into memory mapped userspace memory. For
> this to work drivers encode the correct descriptor blocks and headers
> so that existing PF_PACKET applications work without any modification.
> This only supports the V2 header formats for now. And works by mapping
> a ring of the network device to these slots. Originally I used V2
> header formats but this does complicate the driver a bit.
>
> V3 header formats added bulk polling via socket calls and timers
> used in the polling interface to return every n milliseconds. Currently,
> I don't see any way to support this in hardware because we can't
> know if the hardware is in the middle of a DMA operation or not
> on a slot. So when a timer fires I don't know how to advance the
> descriptor ring leaving empty descriptors similar to how the software
> ring works. The easiest (best?) route is to simply not support this.
>
> It might be worth creating a new v4 header that is simple for drivers
> to support direct DMA ops with. I can imagine using the xdp_buff
> structure as a header for example. Thoughts?
>
> The ndo operations and new socket option PACKET_RX_DIRECT work by
> giving a queue_index to run the direct dma operations over. Once
> setsockopt returns successfully the indicated queue is mapped
> directly to the requesting application and can not be used for
> other purposes. Also any kernel layers such as tc will be bypassed
> and need to be implemented in the hardware via some other mechanism
> such as tc offload or other offload interfaces.
>
> Users steer traffic to the selected queue using flow director,
> tc offload infrastructure or via macvlan offload.
>
> The new socket option added to PF_PACKET is called PACKET_RX_DIRECT.
> It takes a single unsigned int value specifying the queue index,
>
>       setsockopt(sock, SOL_PACKET, PACKET_RX_DIRECT,
> 		&queue_index, sizeof(queue_index));
>
> Implementing busy_poll support will allow userspace to kick the
> drivers receive routine if needed. This work is TBD.
>
> To test this I hacked a hardcoded test into  the tool psock_tpacket
> in the selftests kernel directory here:
>
>       ./tools/testing/selftests/net/psock_tpacket.c
>
> Running this tool opens a socket and listens for packets over
> the PACKET_RX_DIRECT enabled socket. Obviously it needs to be
> reworked to enable all the older tests and not hardcode my
> interface before it actually gets released.
>
> In general this is a rough patch to explore the interface and
> put something concrete up for debate. The patch does not handle
> all the error cases correctly and needs to be cleaned up.
>
> Known Limitations (TBD):
>
>       (1) Users are required to match the number of rx ring
>           slots with ethtool to the number requested by the
>           setsockopt PF_PACKET layout. In the future we could
>           possibly do this automatically.
>
>       (2) Users need to configure Flow director or setup_tc
>           to steer traffic to the correct queues. I don't believe
>           this needs to be changed it seems to be a good mechanism
>           for driving directed dma.
>
>       (3) Not supporting timestamps or priv space yet, pushing
> 	 a v4 packet header would resolve this nicely.
>
>       (5) Only RX supported so far. TX already supports direct DMA
>           interface but uses skbs which is really not needed. In
>           the TX_RING case we can optimize this path as well.
>
> To support TX case we can do a similar "slots" mechanism and
> kick operation. The kick could be a busy_poll like operation
> but on the TX side. The flow would be user space loads up
> n number of slots with packets, kicks tx busy poll bit, the
> driver sends packets, and finally when xmit is complete
> clears header bits to give slots back. When we have qdisc
> bypass set today we already bypass the entire stack so no
> paticular reason to use skb's in this case. Using xdp_buff
> as a v4 packet header would also allow us to consolidate
> driver code.
>
> To be done:
>
>       (1) More testing and performance analysis
>       (2) Busy polling sockets
>       (3) Implement v4 xdp_buff headers for analysis

I like this idea and we should generalize the API that make rx zerocopy 
not specific to packet socket. Then we can make this use for e.g macvtap 
(pass-through mode). But instead of the headers, ndo_ops should support 
refill from non-fixed memory location from userspace (per packet or 
packets) to satisfy the requirement of virtqueues.

Thanks

>       (4) performance testing :/ hopefully it looks good.
>
> Signed-off-by: John Fastabend<john.r.fastabend@...el.com>

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