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Re: [Xen-devel] [3.15-rc3] Bisected: xen-netback mangles packets between two guests on a bridge since merge of "TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy" series.


Sorry for the long silence on this issue, I was busy trying to figure out what went wrong. Fun facts:

- commenting out that _pskb_pull_tail from tx_submit which unconditionally pulls up the linear area to 128 bytes seems to solve the problem - I could repro the problem only when the sending guest had a 64 bit kernel, but then even with 3.2. On the other hand, with 32 bit sending guest it works fine. More exactly I think it boils down to the actual config, I used XenServer Dom0 config files, see them here:
- with 64 bit Debian 7 kernel as sender it also works, so I guess it's not about 32/64 bit, but something in the config
- the receiving guest, where wget ran, doesn't matter.
- the "more than MAX_SKB_FRAGS slots" thing was a red herring. A typical skb layout (on the sender's xenvif_start_xmit) which gets corrupted:
linear area: 66 bytes
0. frag: 52 bytes
1. frag: 1200 bytes
- so I guess the problem is when that pull_tail pulls the whole first frag into the linear area
- a corrupt packet on the receiver side looks like the following:
  - linear buffer: 128 bytes, content is OK
  - the content of the frag area is shifted back 4096 bytes in the
TCP stream. So instead of the Nth byte it starts with the (N-4096)th byte
- the length is the same as on the sender side, I've checked by looking at the IP id fields - otherwise the stream content looks ok (I used a continuously incrementing pattern)
  - the next packet starts at the right place
- the pulling itself doesn't cause the corruption, I've printed out the first frag after that, and it still looks OK - ftrace_printk("%*ph") seems to have problems when the pointer points to a grant mapped page. I have the impression that it tries to dereference it when I read the trace buffer, at which point the mapping and the content is long gone.

I'll continue to look into this next week


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