Re: [Qemu-devel] [PATCH] net/hub: remove can_receive handler

[Stefan Hajnoczi]

On Mon, Oct 21, 2013 at 03:44:46PM +0400, Fedorov Sergey wrote:
> After our discussion about this patch I decided to keep my patch in
> our branch until rebase onto a new release. Recently I have rebased
> our branch onto v1.5.3 and reverted my patch. Then I face an issue
> when using user-mode networking with USB network device for mounting
> root file system through NFS. Fragmented UDP packets from host to
> guest does not handled properly. Seems that some fragments is lost
> or somehow stalled. See guest tcpdump log below.
> 
> 03:16:52.259690 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF],
> proto UDP (17), length 164)
>     10.0.2.15.3369105030 > 10.0.2.2.nfs: 136 readdirplus fh 
> Unknown/0100070004001200000000002873593C9B3C43388E23748B0BAD870C00000000
> 512 bytes @ 0 max 4096 verf 0000000000000000
> 03:16:52.262323 IP (tos 0x0, ttl 64, id 16, offset 0, flags [+],
> proto UDP (17), length 1500)
>     10.0.2.2.nfs > 10.0.2.15.3369105030: reply ok 1472 readdirplus
> POST: DIR 40777 ids 0/0 sz 4096 verf 0000000000000000
> 03:16:52.264592 IP (tos 0x0, ttl 64, id 16, offset 1480, flags [+],
> proto UDP (17), length 1500)
>     10.0.2.2 > 10.0.2.15: udp
> 03:16:54.462961 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF],
> proto UDP (17), length 164)
>     10.0.2.15.3369105030 > 10.0.2.2.nfs: 136 readdirplus fh 
> Unknown/0100070004001200000000002873593C9B3C43388E23748B0BAD870C00000000
> 512 bytes @ 0 max 4096 verf 0000000000000000
> 03:16:54.466300 IP (tos 0x0, ttl 64, id 17, offset 0, flags [+],
> proto UDP (17), length 1500)
>     10.0.2.2.nfs > 10.0.2.15.3369105030: reply ok 1472 readdirplus
> POST: DIR 40777 ids 0/0 sz 4096 verf 0000000000000000
> 03:16:54.467084 IP (tos 0x0, ttl 64, id 17, offset 1480, flags [+],
> proto UDP (17), length 1500)
>     10.0.2.2 > 10.0.2.15: udp
> ...
> 
> I didn't investigate the cause of the problem in detail. I just reverted
> 
> commit 199ee608f0d08510b5c6c37f31a7fbff211d63c4
> Author: Luigi Rizzo <address@hidden>
> Date:   Tue Feb 5 17:53:31 2013 +0100
> 
>     net: fix qemu_flush_queued_packets() in presence of a hub
> 
> And then applied my patch. After that everything works fine for me.
> See guest tcpdump log below.
> 
> 04:45:15.897245 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF],
> proto UDP (17), length 164)
>     10.0.2.15.3642011847 > 10.0.2.2.nfs: 136 readdirplus fh 
> Unknown/0100070004001200000000002873593C9B3C43388E23748B0BAD870C00000000
> 512 bytes @ 0 max 4096 verf 0000000000000000
> 04:45:15.899686 IP (tos 0x0, ttl 64, id 15, offset 0, flags [+],
> proto UDP (17), length 1500)
>     10.0.2.2.nfs > 10.0.2.15.3642011847: reply ok 1472 readdirplus
> POST: DIR 40777 ids 0/0 sz 4096 verf 0000000000000000
> 04:45:15.906253 IP (tos 0x0, ttl 64, id 15, offset 1480, flags [+],
> proto UDP (17), length 1500)
>     10.0.2.2 > 10.0.2.15: udp
> 04:45:15.906687 IP (tos 0x0, ttl 64, id 15, offset 2960, flags
> [none], proto UDP (17), length 240)
>     10.0.2.2 > 10.0.2.15: udp
> 
> So there must be something wrong with already applied patch. What
> could you suggest?

The next step is to investigate the cause.

Perhaps hw/usb/dev-network.c:usb_net_handle_datain() is not calling
qemu_flush_queued_packets() every time in_buf[] is read completely.
This if statement looks strange to me:

if (s->in_ptr >= s->in_len &&
    (is_rndis(s) || (s->in_len & (64 - 1)) || !len)) {
    /* no short packet necessary */
    usb_net_reset_in_buf(s);
}

Try placing printfs to find out whether qemu_flush_queued_packets() is
getting called when you see packet loss.

Stefan