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

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Fri, 9 Sep 2022 07:15:02 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Fri, Sep 09, 2022 at 03:41:54PM +0800, Heng Qi wrote:
> >
> >
> > 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 |
>
> And in fact, B and C both use an offset now, right?


B: offset is used to get the position to place the payload.
C: The offset is used to reserve some space for the device, which the driver can
   use as headroom.

   In order to make the payload page-aligned, we can only hand over the entire
   page to the device, so we cannot reserve some headroom in advance.

>
> > 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;
>
> there's also the cost of the indirect buffer since that is used when
> there is a chain.

Yes


>
> > - 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?
>
> I think either both B and C depending on the mergeable buffers flag,
> or just one of these two.

If I understand correctly, B does not depend on mergeable, while C must depend
on mergeable.

Thanks.


>
> > ---
> >
> > Thanks.
>

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