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 -- 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
