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From: Jason Wang <jasowang@redhat.com>
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Date: Tue, 8 Sep 2015 17:55:14 +0800
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Subject: Re: [Qemu-devel] Segfault using qemu-system-arm in smc91c111
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To: Peter Maydell <peter.maydell@linaro.org>, Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, Peter Crosthwaite <crosthwaitepeter@gmail.com>, Fam Zheng <famz@redhat.com>, qemu-devel <qemu-devel@nongnu.org>



On 09/07/2015 05:21 PM, Peter Maydell wrote:
> CCing the net maintainers on this thread seems like it would
> be a good idea...
>
> On 7 September 2015 at 08:47, Richard Purdie
> <richard.purdie@linuxfoundation.org> wrote:
>> On Sun, 2015-09-06 at 17:48 -0700, Peter Crosthwaite wrote:
>>> This doesn't sound right. There are other network controllers that
>>> rely of can_receive catching all cases properly. Is this a regression?
>>> Looking at logs, I see some refactoring of QEMU net framework around
>>> June timeframe, if you rewind to QEMU 2.3 (or earlier) does the bug go
>>> away?
>> I did find an interesting comment in this commit:
>>
>> http://git.qemu.org/?p=qemu.git;a=commitdiff;h=625de449fc5597f2e1aff9cb586e249e198f03c9
>>
>> """
>> Since commit 6e99c63 "net/socket: Drop net_socket_can_send" and friends,
>> net queues need to be explicitly flushed after qemu_can_send_packet()
>> returns false, because the netdev side will disable the polling of fd.
>> """
>>
>> smc91x111 is calling flush functions when it knows can_receive
>> would/should return false. I believe that is the bug here.
>>
>> I suspect the driver needs:
>>
>> * can_receive to actually return the right value
>> * the locations of the flush calls to be when there is receive space

Yes, please do this.

>> This could explain what changed to break this and why moving the flush
>> calls works in my patch.
> thanks
> -- PMM