Re: [Qemu-devel] Re: 2.6.24 says "serial8250: too much work for irq4" a lot.

[Aurelien Jarno]

On Sun, Feb 10, 2008 at 11:26:06AM +0000, Paul Brook wrote:
> On Sunday 10 February 2008, Blue Swirl wrote:
> > On 2/9/08, H. Peter Anvin <address@hidden> wrote:
> > > Blue Swirl wrote:
> > > >> If you look at the patch, there are no timing dependencies; the only
> > > >> parameter is the depth of the virtual queue.  The exhaustion is
> > > >> completely controlled by target OS access patterns.
> > > >
> > > > Thanks, this clarified the difference. But I'll rephrase my original
> > > > comment:
> > > >
> > > > The patch looks OK, but the simulated FIFO exhaustion should benefit
> > > > all devices, as
> > > > discussed here:
> > > > http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html
> > >
> > > The difference is you *can't* do that in a general layer.
> >
> > What makes you think that is impossible? 
> 
> IIUC the proposed patch makes the serial driver return an empty FIFO exactly 
> once, them immediately continue receiving data. Throughput should be 
> approximately the same, you've just got a bit of extra overhead to process 
> the additional interrupts.  This is very different to the previous patch 
> which did time-based throughput limiting.
> 
> You can't do this in generic code because there's no way to guess when the 
> guest os has seen the FIFO empty condition. The best you can do is pause for 
> some arbitrary length of time, which is both unreliable (the guest OS may not 
> have got to far enough yet, especially if the host machine is heavily 
> loaded), and has a significant negative impact on throughput.
> 
> > Also win2k install hack in ide.c seems to be related to this problem,
> > so even more generic solution would be desirable.
> 
> IIUC the win2k hack is an actual timing problem. The win2k IDE drivers are 
> buggy, and fall over if the drive responds too soon.
> 

Is everybody convinced about this patch now? I would really like to see
it in the CVS, as it greatly improves the usability of the serial 
consoles on targets running GNU/Linux (and probably OSes).

-- 
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