[Qemu-devel] Re: [PATCH] PPC: tell the guest about the time base frequency

[Blue Swirl]

On Fri, Jan 8, 2010 at 5:43 PM, Alexander Graf <address@hidden> wrote:
> Our guest systems need to know by how much the timebase increases every 
> second,
> so there usually is a "timebase-frequency" property in the cpu leaf of the
> device tree.
>
> This property is missing in OpenBIOS, as is the "clock-frequency" property 
> that
> tells the guest how fast the CPU is. FWIW that one is only used for
> /proc/cpuinfo though.
>
> With qemu, Linux's fallback timebase speed and qemu's internal timebase speed
> match up. With KVM, that is no longer true. The guest is running at the same
> timebase speed as the host.
>
> This leads to massive timing problems. On my test machine, a "sleep 2" takes
> about 14 seconds with KVM enabled.
>
> This patch exports the timebase and clock frequencies to OpenBIOS, so it can
> then put them into the device tree. I'll push the OpenBIOS change with the
> NewWorld patch set, once that's either been reviewed or applied.

IIRC copying the host CPU frequency to guest was rejected earlier for x86.