Re: The SMP RHEL 5.1 PAE guest can't boot up issue

[Avi Kivity <avi@qumranet.com>]

Marcelo Tosatti wrote:
> Another source of problems in this area is that the TSC_OFFSET is
> initialized to represent zero at different times for VCPU0 (at boot) and
> the remaining ones (at APIC_DM_INIT).
>
>   

I added tsc sync in the guest bios some time ago, so this should be 
solved now.

>> This will improve tsc quality for those machines, but we can't depend on 
>> it, since some machines don't have constant tsc.  Further, I don't think 
>> really large machines can have constant tsc since clock distribution 
>> becomes difficult or impossible.
>>     
>
> As discussed earlier, in case the host kernel does not have the TSC
> stable, it needs to enforce a state which the guest OS will not trust
> the TSC. The easier way to do that is to fake a C3 state. However, QEMU
> does not emulate IO port based wait. This appears to be the reason for
> the high-CPU-usage-on-idle with Windows guests, fixed by disabling C3
> reporting on rombios (commit cb98751267c2d79f5674301ccac6c6b5c2e0c6b5 of
> kvm-userspace).
>
>   

Oh.  Can you point me at documentation for the io port wait thing?

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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