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

[Blue Swirl]

On Fri, Jan 8, 2010 at 6:07 PM, Alexander Graf <address@hidden> wrote:
>
> On 08.01.2010, at 19:04, Blue Swirl wrote:
>
>> 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.
>
> Well IIRC x86 Linux tries to find out the cpu frequency itself.
> PPC Linux doesn't - it completely relies on entries in the device tree.

The frequency could be a parameter for the -cpu flag, like -cpu
970fx,frequency=1000000000.