[re-sending since the original message was bounced back, something
about HTML subpart]
Hi,
I'm playing with the PACKET_RX_RING option to packet sockets and I
noticed that the status field from the tpacket_hdr structures inside
the mmaped buffer is actually a bit mask. I'm testing with 4Gbps
traffic rates and sometimes I get status=5. I'm using Ubuntu 12.04
with a 3.8.0-29 kernel, and at least from the code (I got it using
"sudo apt-get source linux-image-$(uname -r)") it seems the 5 means
TP_STATUS_USER | TP_STATUS_LOSING. The 3.8.0 kernel code has this line
in tpacket_rcv():
/*
* LOSING will be reported till you read the stats,
* because it's COR - Clear On Read.
* Anyways, moving it for V1/V2 only as V3 doesn't need this
* at packet level.
*/
if (po->stats.tp_drops)
status |= TP_STATUS_LOSING;
Also, in some source code example I found using google
(http://www.google.ro/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&ved=0CEIQFjAE&url=http%3A%2F%2Fstuff.mit.edu%2Fafs%2Fsipb%2Fcontrib%2Flinux%2Ftools%2Ftesting%2Fselftests%2Fnet%2Fpsock_fanout.c&ei=lM8MU5yhB-iK7Aan7IGgAQ&usg=AFQjCNFtZBHSWgzHZkV3LNfLOSSTq0pV4Q&sig2=razCT0kUDv2-BIm_8gTE5g&bvm=bv.61725948,d.ZGU),
the status is checked like this:
while (header->tp_status & TP_STATUS_USER && count < RING_NUM_FRAMES) {
[...]
I haven't yet figured out how TP_STATUS_LOSING works, but I think it
would be worthwhile to at least document that the status is a bit mask
and not a value - that's what the patch at the end does.
Michael, maybe you can Cc somebody more knowledgeable on this to
confirm? IIRC there were some message exchanges on this on the list
recently...
diff --git a/man7/packet.7 b/man7/packet.7
index 1d3f222..6bac465 100644
--- a/man7/packet.7
+++ b/man7/packet.7
@@ -353,9 +353,9 @@ The packet socket owns all slots with status
.BR TP_STATUS_KERNEL .
After filling a slot, it changes the status of the slot to transfer
ownership to the application.
-During normal operation, the new status is
-.BR TP_STATUS_USER ,
-to signal that a correctly received packet has been stored.
+During normal operation, the new status has the
+.BR TP_STATUS_USER
+bit set to signal that a correctly received packet has been stored.
When the application has finished processing a packet, it transfers
ownership of the slot back to the socket by setting the status to
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