Re: [PATCH v1 0/5] KVM in-guest performance monitoring

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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).

--
error compiling committee.c: too many arguments to function

--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html


[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]
  Powered by Linux