[Xen-devel] confusion on front end driver and allox_rx_buffers.

["Ronald G. Minnich" <rminnich@xxxxxxxx>]

In the frontend ethernet driver, there is a ring buffer of buffers 
pointers -- skbs are used on linux. 

np->rx_resp_cons is I think the number that says which buffer we are 
consuming, and increases from 0 -> infinity. 
np->rx->req_prod is Xen's count of which buffer it has produced, and it 
also increase from 0 -> infinity. Driver uses good old fashioned 
power-of-two ring sizes and masks to to turn these counts into indices in 
the ring. 

Your goal is to consume buffers from the rx ring as Xen fills them. You 
want to try to catch np->rx_resp_cons to np->rx->req_prod (I assume). You 
get interrupts as buffers become available, i.e. are filled in by Xen. 

Initially, network startup calls network_alloc_rx_buffers to set up the 
ring with a set of skbs, and hands MFNs of these skbs to Xen in a shared 
array that implements the ring buffer. 

Initially all counters are zero (cons and prod).

Pretend we get a packet and netif_poll gets called. I expect (?) that
np->rx->req_prod is 1. netif_poll will eat the buffer, and now the ring
has had a buffer consumed, so netif_poll will call
network_alloc_rx_buffers to refill the ring.

I see this:

    do {
        skb = dev_alloc_skb(RX_BUF_SIZE);
        .
        .
        .
        id = GET_ID_FROM_FREELIST(np->rx_skbs);

        np->rx_skbs[id] = skb;
        
        np->rx->ring[MASK_NETIF_RX_IDX(i)].req.id = id;
        .
        .
        nr_pfns++;
    }
    while ( (++i - np->rx_resp_cons) != NETIF_RX_RING_SIZE );

OK, the question: I only consumed one buffer, why is this code refilling 
the whole ring this way? What's happening to me is that the id = 
GET_ID_FROM_FREELIST is failing badly because the do loop is running from 
2 to NETIF_RX_RING_SIZE, which is wrong. 

I think I'm getting caught between the differences in Plan 9 and Linux net 
interrupt handling, which I don't fully understand yet, but some of this 
ether xen front end code has me confused. 

Comments and clarification are most welcome :-)

ron
p.s. and yes, I am at the DHCP step and it's damn near working. 


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