Re: [PATCH] fix reading from character devices

[Dor Laor <dor.laor-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Mon, 2008-01-21 at 12:50 +0200, 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?
> 

Virtio+tap with the performance hack does it's own select on the related
fd. Virtio+user will remain as before.



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