Re: [kvm-ppc-devel] [Kvm-ppc-devel] user-accessible SPRs

[Hollis B. <ho...@us...>]

On Fri, 2007-08-10 at 08:28 -0700, Yoder Stuart-B08248 wrote:
> 
> It's not mode related, but in BookE, TBU/TBL are User read / Supervisor
> write.  So we won't be able to trap/emulate reads to the timebase on
> real Book E hardware if we think the timebase needs to be virtualized
> for some reason.  Don't think this is a big deal.

Agreed. Practically speaking, it actually is pretty important that
guests be able to tell how much real time has elapsed. In Linux's case
this is where we replay ticks that would have triggered when the VCPU
was de-scheduled.

Of course, I have no idea how an RTOS would react to missing its
decrementer/FIT interrupts. Hopefully we can mitigate that problem by
setting appropriate host scheduling parameters in the first place...

If we want to consider live migration in the future, the user-accessible
timebase will raise some issues... especially if the two systems'
timebase frequencies are not equal.

-- 
Hollis Blanchard
IBM Linux Technology Center