Re: [Qemu-ppc] [PATCH 1.7 v2 2/2] PPC: BookE: Make FIT/WDT timers at best millisecond grained

[Tom Musta]

On 11/25/2013 3:46 PM, Alexander Graf wrote:
> The default granularity for the FIT timer on 440 is on every 0x1000th
> transition of TB from 0 to 1. Translated that means 48828 times a second.
> 
> Since interrupts are quite expensive for 440 and we don't really care
> about the accuracy of the FIT to that significance, let's force FIT and
> WDT to at best millisecond granularity.
> 
> This basically restores behavior as it was in QEMU 1.6, where timers
> could only deal with millisecond granularities at all.
> 
> This patch greatly improves performance with the 440 target and restores
> roughly the same performance level that QEMU 1.6 had for me.
> 

@Reviewed-by: Tom Musta <address@hidden>

Alexander:

Is the problem the default?  If so, would it be better to change the default?
(for example, set the default TCR_FP for the 440 to 0b10).  Of course, if your
software is setting this back to 0b00, you will still have a problem.

The downside to your approach is that it eliminates the possibility for fine
grained timer interrupts for all Book E implementations and all kernels and/or
hypervisors.  I am not aware of any such system so my comment is purely
theoretical.

Regards,
Tom