[Xen-devel] GPLPV: Respecting SG capability

[Russ Blaine <russell.blaine@xxxxxxx>]

Hi James,

As you may have heard, the latest GPLPV release doesn't work on 
Opensolaris dom0. Our backend net driver doesn't support scatter/gather, 
but it seems that GPLPV now requires it.
I have a fix for this in the frontend which coalesces all NDIS buffers 
into one ring transaction. With the fix, packets flow again.
Once this is addressed I may go and implement SG in our backend anyway, 
but I wanted to get this fix into the GPLPV source first to enable 
networking on older and current dom0s.
The fix as I have it now is around line 229 of xennet_tx.c (see below). 
I think a further and necessary improvement on this would be to avoid 
the construction of the header_buf() altogether in the no-sg case. There 
is also only one tx_sendbuf per driver instance (it just points to 
tx_hb[0]), and I suppose there should be several and that they should be 
managed in the same way you deal with the tx_hb[] instances.
Actually, on second look, there is a per-driver-instance tx_lock which 
must act to serialize all transmits? In which case we only need a single 
tx_sendbuf anyhow. Would Windows benefit from having a reentrant send 
routine?
Here is the prototype of the fix.

 if (xi->config_sg == 0) {
      int i;
      ULONG len;
      ULONG offset = 0;
      PNDIS_BUFFER buf;

      buf = pi.first_buffer;
      while (buf) {
          PUCHAR src_addr;

          NdisQueryBufferSafe(buf, &src_addr, &len, NormalPagePriority);

          memcpy((PUCHAR)xi->tx_sendbuf.virtual + offset, src_addr, len);
          offset += len;

          NdisGetNextBuffer(buf, &buf);
      }

      tx0->gref = (grant_ref_t)xi->tx_sendbuf.logical.QuadPart >> 
PAGE_SHIFT;
      tx0->offset = (USHORT)xi->tx_sendbuf.logical.LowPart & (PAGE_SIZE 
- 1);
      ASSERT(offset == pi.total_length);
      tx0->size = offset;
      tx0->flags &= ~NETTXF_more_data;
      sg_element = sg->NumberOfElements;
  } else if (header_buf)

Cheers,
- Russ

-----------------------------------------------------
Russ Blaine | Solaris Kernel | russell.blaine@xxxxxxx

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