On 05/12/2011 04:06 PM, Joerg Roedel wrote:
On Thu, May 12, 2011 at 12:51:11PM +0300, Avi Kivity wrote: > On 05/12/2011 12:33 PM, Joerg Roedel wrote: >> Gaah, I was just about to submit a talk about PMU virtualization for KVM >> Forum :) > > Speed matters. I'll take that as an argument for paravirt pmu, because that one is certainly faster than anything we can emulate on-top of perf_events ;-)
Correct, though some massaging of perf_events can make it faster. Still we'll pay with more exits with the architectural PMU.
Note a v2 PMU can reduce the exit count since it has MSRs for programming many PMCs at once.
> Note, at this time the architectural PMU is only recognized on an Intel > host. > > Is the statement "if an AMD processor returns non-zero information in > cpuid leaf 0xa, then that processor will be compatible with other > vendors' processors reporting the same information" correct? AMD processors don't implement that cpuid leaf.
Right. But if an AMD processor were to implement that leaf, it would be in a compatible manner, yes?
That allows us to - if (vendor == intel && leaf_0xa_indicates_arch_pmu) + if (leaf_0xa_indicates_arch_pmu)
> If so, we can move the detection of the architectural pmu outside the > cpu vendor checks, and this code will work on both AMD and Intel > processors (even if the host cpu doesn't have an architectural PMU). Thats already some kind of paravirtualization. Don't get me wrong, I see the point of emulating a real pmu in the guest. But on the other side I think a interface that works across cpu models fits better into the KVM design, because KVM (oposed to other hypervisors) trys to hide details of the host cpu as much as necessary to get migration working between different cpus. And since pmu are, as you said, very model-specific, some abstraction is needed.
The architectural PMU is not model specific.
In the end probably both ways can be implemented in parallel: * re-implementing the host-pmu using perf_events for -cpu host guests * a paravirt pmu for everybody that wants migration and more accurate results
A paravirt PMU also has to be implemented on top of perf_events. Otherwise we can't share this resource. So the only question is what the interface looks like. The arch pmu is non-optimized, but well specified and somewhat supported in guests. A paravirt pmu is not so well specified at this point but can be faster (less exits).
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