Re: [Qemu-devel] cpufreq and QEMU guests

[Alexander Graf]

Am 16.09.2013 um 07:15 schrieb Benoît Canet <address@hidden>:

> 
> Hello,
> 
> I know a cloud provider worried about the fact that the /proc/cpuinfo of his
> guests give a bogus frequency to his customer.
> 
> QEMU and the guests kernel currently have no way to reflect the host frequency
> changes to the guests.
> 
> The customer compute intensive application then read this information and take
> wrong decisions.

Why do they care about the frequency? Is it for scheduling workloads? The only 
other case I can think of would be the TSC and that should be fixed frequency 
these days.

If it's scheduling, you could maybe expose the unavailable compute time as 
steal time to the guest. Exposibg frequency in a virtual environment feels 
backwards.

Alex

> 
> I looked at the various Linux cpufreq drivers and they all seems to be table
> based. Is it true ?
> 
> For example the acpi cpufreq driver have 16 differents pstates at hand to 
> lookup
> in the pstate table and get the frequency.
> 
> Given that guests can migrate from one hardware to a slightly different 
> hardware
> the table may become wrong after live migration.
> 
> What would be the best hardware to emulate in order to pass an arbitrary
> frequency to the guest ?
> 
> Would a pvfreq paravirtualized QEMU hardware and a guest driver implementing
> only the callbacks needed to read the frequency be a good idea ?
> 
> Best regards
> 
> Benoît
> 
> ps:
> I CC this mail to the other QEMU arch maintainers because the problem must be
> the same everywhere where KVM run.