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

[Blue Swirl]

On Fri, Jan 8, 2010 at 6:27 PM, Alexander Graf <address@hidden> wrote:
>
> On 08.01.2010, at 19:22, Blue Swirl wrote:
>
>> 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.
>
> We could implement that as an override to the current static code path.
> For KVM it doesn't make any sense, as you really want to see the host 
> frequency in the guest here. That's what users expect. Period.

Even with 10 guests? What about migration?