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


From: Alexander Graf
Subject: [Qemu-devel] Re: [PATCH] PPC: tell the guest about the time base frequency
Date: Fri, 8 Jan 2010 19:07:16 +0100

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.

Alex



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