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

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Tue, 2010-06-22 at 09:58 +0200, Jes Sorensen wrote:
> 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.
That doesn't resolve the issue that guest os software has to poll register.