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

[Fedorov Sergey <s.fedorov@samsung.com>]

On 10/29/2013 06:55 PM, Stefan Hajnoczi wrote:
> 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 <rizzo@iet.unipi.it>
>> 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
>

Seems that I have figured out the problem. net_hub_flush() does not 
flush source port. And qemu_flush_queued_packets() also returns after 
calling net_hub_flush(). So I think the problem is that neither 
qemu_flush_queued_packets() nor net_hub_flush() call 
qemu_net_queue_flush() for the source port. So I think it sould be fixed 
in qemu_flush_queued_packets() by removing the return statement after 
calling net_hub_flush(). That fix does work for me. So I could submit 
that patch after getting permission for that.

-- 
Best regards,
Sergey Fedorov, Junior Software Engineer,
Samsung R&D Institute Rus.
E-mail: s.fedorov@samsung.com