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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 14, 2022 at 11:34:43AM +0800, Jason Wang wrote:
> 
> å 2022/9/9 20:38, Xuan Zhuo åé:
> > 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:
> > > > 
> > > > å 2022/9/5 äå4:27, Michael S. Tsirkin åé:
> > > > > 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.
> 
> 
> For C, it might be better to do some tweak since mergeable buffer doesn't
> forbid using a descriptor chain as a single buffer.
> 
> So if it's a descriptor chain we got back the method A by placing the
> payload in a dedicated buffer. If it's not placing the payload in an
> adjacent buffer.
> 
> Thanks

Let's find a way so devices do not care how descriptors are laid out.

> 
> > 
> > > > 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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