Re: [PATCH V2 1/5] ara virt interface of perf to support kvm guestos statistics collection in guest os

[Jes Sorensen]

On 06/22/10 09:47, Zhang, Yanmin wrote:
> On Tue, 2010-06-22 at 09:14 +0200, Jes Sorensen wrote:
>> On 06/22/10 03:49, Zhang, Yanmin wrote:
>>> On Mon, 2010-06-21 at 14:45 +0300, Avi Kivity wrote:
>>> So I think above discussion is around how to expose PMU hardware to guest os. I will
>>> also check this method after the para virt interface is done.
>>
>> You should be able to expose the counters as read-only to the guest. KVM
>> allows you to specify whether or not a guest has read, write or
>> read/write access. If you allowed read access of the counters that would
>> safe a fair bit of hyper calls.
> Thanks. KVM is good in register access permission configuration. But things are not so
> simple like that if we consider real running environment. Host kernel might schedule
> guest os vcpu thread to other cpus, or other non-kvm processes might preempt the vcpu
> thread on this cpu.
> 
> To support such capability you said, we have to implement the direct exposition of PMU
> hardware to guest os eventually.

If the guest is rescheduled to another CPU, or you get a preemption, you
have a VMEXIT. The vcpu thread will not migrate while it is running, so
you can handle it while the the VMEXIT is being serviced.

Exposing the counters read-only would save a lot of overhead for sure.

>> Question is if it is safe to drop overflow support?
> Not safe. One of PMU hardware design objectives is to use interrupt or NMI to notify
> software when event counter overflows. Without overflow support, software need poll
> the PMU registers looply. That is not good and consumes more cpu resources.

Here is an idea, how about having the overflow NMI in the host trigger a
flag that causes the PMU register read to trap and get special handling?
That way you could propagate the overflow back down to the guest.

> Besides the para virt perf interface, I'm also considering the direct exposition
> of PMU hardware to guest os. But that will be another very different implementation. We
> should not combine it with pv interface. Perhaps our target is to implement both, so
> unmodified guest os could get support on perf statistics.

That was what I was looking at initially, but it got stalled. I think it
will make sense to build it on top of the infrastructure you have
already posted, so once that settles it will definitely be easier to do.

Cheers,
Jes
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