Re: [PATCH] fix reading from character devices

[Jan Kiszka <jan.kiszka-kv7WeFo6aLtBDgjK7y7TUQ@public.gmane.org>]

Avi Kivity wrote:
> Dor Laor wrote:
>> On Mon, 2008-01-21 at 12:13 +0200, Avi Kivity wrote:
>>  
>>> Jan Kiszka wrote:
>>>    
>>>> Hi Avi,
>>>>
>>>> commit "kvm: qemu: consume all pending I/O in I/O loop"
>>>> (8ab8bb09f1115b9bf733f885cc92b6c63d83f420) broke reading data bursts
>>>> from serial devices (and maybe from other character devices as well) by
>>>> guests. Reason: serial devices do input flow control via fd_read_poll,
>>>> but qemu now ignores this fact by pushing all data into the virtual
>>>> device as soon as it is available.
>>>>
>>>> Patch below is not really nice (just as the whole internal virtual I/O
>>>> interface at the moment, IMHO), but it re-enables the serial ports for
>>>> now.
>>>>
>>>>         
>>> I'm worried that it will break Dor's hack that speeds up virtio.  Dor?
>>>
>>>     
>>
>> It should be fine. Tap device without my hack has fd_read_poll null and
>> I hacked it to have a handler that returns false if virtio is used and
>> true otherwise. So Jan's patch should set 'more' to 1 and it will be
>> like before.
>>
>>   
> 
> But if virtio is used with this patch, it won't set 'more' to 1.  Will
> virtio handle it or will throughput drop to be related to whatever
> timers we have set up?

As long as virtio_net_can_receive returns 1, the loop will keep on
feeding also this device. And if it returns 0, I see no reason for
ignoring this signal. Anythings else would be, well, weird. However, the
current io-polling loop in git urgently needs fixing, or even a redesign.

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux

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