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[Qemu-devel] cpufreq and QEMU guests

From: Benoît Canet
Subject: [Qemu-devel] cpufreq and QEMU guests
Date: Mon, 16 Sep 2013 14:15:46 +0200
User-agent: Mutt/1.5.21 (2010-09-15)


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.

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


I CC this mail to the other QEMU arch maintainers because the problem must be
the same everywhere where KVM run.

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