[Qemu-devel] [QEMU RFC 0/2] Spying on Memory to implement ethernet can_recieve()

[Peter Crosthwaite]

Hi All,

Have a bit of a tricky question about ethernet controllers. We are maintaining 
two ethernet controllers the cadence GEM and the Xilinx AXI Ethernet both of 
which are scatter gather (SG) DMA capable. The issue comes about when trying to 
impelement the can_recieve() function for each.

For the sake of background, the ethernet can_recieve() function is used by net 
devices to signal to the net API that it cant consume data. As QEMU network 
interfaces are infinately faster than real hardware you need to implement this 
function to insulate against mass packet droppage. The can_recieve has no real 
hardware analog, the network devices would just drop the packets (but the 
packets would be comming at a much slower rate).

For most devices, the return of this function is dependant on internal state of 
the device. However, for SGDMA, whether or not you can receive is dependent on 
the state of the SGDMA buffer descriptors, which are stored in RAM. So to 
properly implement can_receive() we need to spy on the contents of memory, to 
see if the next descriptor is valid. This is not too hard. The hard part is 
monitoring the descriptor for change after you have returned false from can 
recieve() so you can tell the net layer you are now ready. Essentially, you 
need to watch memory and wakup when the cpu writes to the magic address. Patch 
1 is a hackish first attempt at this for the cadence GEM, using the Memory API.

The alternative to this is to impelement timing for the ethernet device, which 
is truer to the real hardware. Throttle the ethernet trasffic based on wire 
speed, not the state of the ethernet controller. Ive had an attempt at this for 
the AXI Ethernet (Patch 2) but a proper solution to this would be to implement 
this wire side (in the net layer) rather than device side.

Peter Crosthwaite (2):
  cadence_gem: Throttle traffic using buffer state
  xilinx_axienet: Model timing.

 hw/cadence_gem.c    | 75 ++++++++++++++++++++++++++++++++++++++++++++---------
 hw/xilinx_axienet.c | 16 ++++++++++--
 2 files changed, 77 insertions(+), 14 deletions(-)

-- 
1.7.12.1.396.g16eed7c