Re: [Qemu-devel] [PATCH] USB 2.0 EHCI emulation

[Avi Kivity]

Paul Brook wrote:
> On Tuesday 08 January 2008, Dor Laor wrote:
>   
> > On Tue, 2008-01-08 at 01:30 +0000, Paul Brook wrote:
> >     
> > > > -The host kernel was configured with dynamic tick & hi-res timers, to
> > > > allow the desired timer resolution. USB 2.0 microframe is 125usec.
> > > >         
> > It still works even without accurate timing demands.
> > Only isochronous mode will have problems and it is not yet supported for
> > ehci.
> >     
>
> It could also cause problems for periodic interrupt transfers. It's not 
> uncommon for linux hosts to have a minimum timer period of 10ms (100Hz). This 
> means the periodic list will be traversed 80x slower than it should, so a 
> typical for a mouse or tablet with a 10ms poll interval will only be polled 
> every 800ms. 800ms lag on a mouse is unacceptable.
>
> The existing USB hosts have similar issues. However the problem is an order of 
> magnitude less severe, so isn't noticeable under normal circumstances.
>
>   

Surely qemu should read the time and adjust?  For example, a 1000Hz 
guest running on a 100Hz host will see a 10x slowdown in real time.

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