Re: [Xen-devel] [RFC] netif: staging grants for requests

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Joao Martins [mailto:joao.m.martins@xxxxxxxxxx]
> Sent: 14 December 2016 18:11
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>; Andrew Cooper
> <Andrew.Cooper3@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; Paul Durrant
> <Paul.Durrant@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Subject: [RFC] netif: staging grants for requests
> 
> Hey,
> 
> Back in the Xen hackaton '16 networking session there were a couple of ideas
> brought up. One of them was about exploring permanently mapped grants
> between
> xen-netback/xen-netfront.
> 
> I started experimenting and came up with sort of a design document (in
> pandoc)
> on what it would like to be proposed. This is meant as a seed for discussion
> and also requesting input to know if this is a good direction. Of course, I
> am willing to try alternatives that we come up beyond the contents of the
> spec, or any other suggested changes ;)
> 
> Any comments or feedback is welcome!
> 

Hi,

  Sorry for the delay... I've been OOTO for three weeks.

  I like the general approach or pre-granting buffers for RX so that the 
backend can simply memcpy and tell the frontend which buffer a packet appears 
in but IIUC you are proposing use of a single pre-granted area for TX also, 
which would presumably require the frontend to always copy on the TX side? I 
wonder if we might go for a slightly different scheme...

  The assumption is that the working set of TX buffers in the guest OS is 
fairly small (which is probably true for a small number of heavily used sockets 
and an OS that uses a slab allocator)...

  The guest TX code maintains a hash table of buffer addresses to grant refs. 
When a packet is sent the code looks to see if it has already granted the 
buffer and re-uses the existing ref if so, otherwise it grants the buffer and 
adds the new ref into the table.

  The backend also maintains a hash of grant refs to addresses and, whenever it 
sees a new ref, it grant maps it and adds the address into the table. Otherwise 
it does a hash lookup and thus has a buffer address it can immediately memcpy 
from.

  If the frontend wants the backend to release a grant ref (e.g. because it's 
starting to run out of grant table) then a control message can be used to ask 
for it back, at which point the backend removes the ref from its cache and 
unmaps it.

  Using this scheme we allow a guest OS to still use either a zero-copy 
approach if it wishes to do so, or a static pre-grant... or something between 
(e.g. pre-grant for headers, zero copy for bulk data).

  Does that sound reasonable?

    Paul
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