On 2017年04月21日 21:08, Vlad Yasevich wrote:
On 04/21/2017 12:05 AM, Jason Wang wrote:
On 2017年04月20日 23:34, Vlad Yasevich wrote:
On 04/17/2017 11:01 PM, Jason Wang wrote:
On 2017年04月16日 00:38, Vladislav Yasevich wrote:
Curreclty virtion net header is fixed size and adding things to it is rather
difficult to do. This series attempt to add the infrastructure as well as some
extensions that try to resolve some deficiencies we currently have.
First, vnet header only has space for 16 flags. This may not be enough
in the future. The extensions will provide space for 32 possbile extension
flags and 32 possible extensions. These flags will be carried in the
first pseudo extension header, the presense of which will be determined by
the flag in the virtio net header.
The extensions themselves will immidiately follow the extension header itself.
They will be added to the packet in the same order as they appear in the
extension flags. No padding is placed between the extensions and any
extensions negotiated, but not used need by a given packet will convert to
trailing padding.
Do we need a explicit padding (e.g an extension) which could be controlled by each side?
I don't think so. The size of the vnet header is set based on the extensions negotiated.
The one part I am not crazy about is that in the case of packet not using any extensions,
the data is still placed after the entire vnet header, which essentially adds a lot
of padding. However, that's really no different then if we simply grew the vnet header.
The other thing I've tried before is putting extensions into their own sg buffer, but that
made it slower.h
Yes.
For example:
| vnet mrg hdr | ext hdr | ext 1 | ext 2 | ext 5 | .. pad .. | packet data |
Just some rough thoughts:
- Is this better to use TLV instead of bitmap here? One advantage of TLV is that the
length is not limited by the length of bitmap.
but the disadvantage is that we add at least 4 bytes per extension of just TL data. That
makes this thing even longer.
Yes, and it looks like the length is still limited by e.g the length of T.
Not only that, but it is also limited by the skb->cb as a whole. So adding putting
extensions into a TLV style means we have less extensions for now, until we get rid of
skb->cb usage.
- For 1.1, do we really want something like vnet header? AFAIK, it was not used by modern
NICs, is this better to pack all meta-data into descriptor itself? This may need a some
changes in tun/macvtap, but looks more PCIE friendly.
That would really be ideal and I've looked at this. There are small issues of exposing
the 'net metadata' of the descriptor to taps so they can be filled in. The alternative
is to use a different control structure for tap->qemu|vhost channel (that can be
implementation specific) and have qemu|vhost populate the 'net metadata' of the descriptor.
Yes, this needs some thought. For vhost, things looks a little bit easier, we can probably
use msg_control.
We can use msg_control in qemu as well, can't we?
AFAIK, it needs some changes since we don't export socket to userspace.
It really is a question of who is doing
the work and the number of copies.
I can take a closer look of how it would look if we extend the descriptor with type
specific data. I don't know if other users of virtio would benefit from it?
Not sure, but we can have a common descriptor header followed by device
specific meta data. This probably need some prototype benchmarking to
see the benefits first.
Thanks
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