[virtio-dev] Re: [PATCH v7] virtio_net: support split header

[Heng Qi <hengqi@linux.alibaba.com>]

在 2022/9/5 上午4:27, Michael S. Tsirkin 写道:
> On Fri, Sep 02, 2022 at 03:36:25PM +0800, Heng Qi wrote:
>> We need to clarify that the purpose of header splitting is to make all payloads
>> can be independently in a page, which is beneficial for the zerocopy
>> implemented by the upper layer.
> absolutely, pls add motivation.
>
>> If the driver does not enforce that the buffers submitted to the receiveq MUST
>> be composed of at least two descriptors, then header splitting will become meaningless,
>> or the VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER feature should not be negotiated at this time.
>>
>>
>> Thanks.
>>
>>
>
> This seems very narrow and unecessarily wasteful of descriptors.
> What is wrong in this:
>
> <header>...<padding>... <beginning of page><data>
>
> seems to achieve the goal of data in a separate page without
> using extra descriptors.
>
> thus my proposal to replace the requirement of a separate
> descriptor with an offset of data from beginning of
> buffer that driver sets.
>
>
We have carefully considered your suggestion.

We refer to spec v7 and earlier as scheme A for short. Review scheme A 
below:

| receive buffer |

| 0th descriptor | 1th descriptor |

| virtnet hdr | mac | ip hdr | tcp hdr|<-- hold -->| payload |

We use a buffer plus a separate page when allocating the receive

buffer. In this way, we can ensure that all payloads can be

independently in a page, which is very beneficial for the zerocopy

implemented by the upper layer.

scheme A better solves the problem of headroom, tailroom and memory 
waste, but as you said, this solution relies on descriptor chain.

Our rethinking approach is no longer based on or using descriptor chain.

We refer to your proposed offset-based scheme as scheme B:

As you suggested, scheme B gives the device a buffer, using offset to 
indicate where to place the payload like this:

<header>...<padding>... <beginning of page><data>

But how to apply for this buffer? Since we want the payload to be placed 
on a separate page, the method we consider is to directly apply to the 
driver for two pages of contiguous memory.

Then the beginning of this contiguous memory is used to store the 
headroom, and the contiguous memory after the headroom is directly 
handed over to the device. similar to the following:

<------------------------------------------ receive buffer(2 pages) 
----------------------------------------->

<<---------------------------------- first page 
-----------------------------------><---- second page ------>>

<<Driver reserved, the later part is filled><vheader><transport 
header>..<padding>..<beginning of page><data>>

Based on your previous suggestion, we also considered another new scheme C.

This scheme is implemented based on mergeable buffer, filling a separate 
page each time.

If the split header is negotiated and the packet can be successfully 
split by the device, the device needs to find at least two buffers, 
namely two pages, one for the virtio-net header and transport header, 
and the other for the data payload. Like the following:

| receive buffer1(page) | receive buffer2 (page) |

| virtnet hdr | mac | ip hdr | tcp hdr|<-- hold -->| payload |

At the same time, if XDP is considered, then the device needs to add 
headroom at the beginning of receive buffer1 when receiving packets, so 
that the driver can process programs similar to XDP. In order to solve 
this problem, can scheme C introduce an offset, which requires the 
device to write data from the offset position to receive buffer1, like 
the following:

| receive buffer (page) | receive buffer (page) |

| <-- offset(hold) --> | virtnet hdr | mac | ip hdr | tcp hdr|<-- hold 
-->| payload |

Then we simply compare the advantages and disadvantages of scheme A(spec 
v7), scheme B (offset buffer(2 pages)) and scheme C (based on mergeable 
buffer):

1. desc chain:

- A depends on desciptor chain; - B, C do not depend on desciptor chain.

2. page alloc

- B fills two consecutive pages, which causes a great waste of memory 
for small packages such as arp; - C fills a single page, slightly better 
than B.

3. Memory waste:

- The memory waste of scheme A is mainly the 0th descriptor that is 
skipped by the device; - When scheme B and scheme C successfully split 
the header, there is a huge waste of the first page, but the first page 
can be quickly released by copying.

4. headroom

- The headrooms of plan A and plan B are reserved; - Scheme C requires 
the driver to set off to let the device skip off when using receive buffer1.

5. tailroom

- When splitting the header, skb usually needs to store each independent 
page in the non-linear data area based on shinfo. - The tailroom of 
scheme A is reserved by itself; - Scheme B requires the driver to set 
the reserved padding area for the first receive buffer(2 pages) to use 
shinfo when the split header is not successfully executed; - Scheme C 
requires the driver to set max_len for the first receive buffer(page).


Which plan do you prefer?

---

Thanks.


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