Re: Some Code for Performance Profiling

[Avi Kivity <avi@redhat.com>]

On 03/31/2010 07:53 PM, Jiaqing Du wrote:
> Hi,
>
> We have some code about performance profiling in KVM. They are outputs
> of a school project. Previous discussions in KVM, Perfmon2, and Xen
> mailing lists helped us a lot. The code are NOT in a good shape and
> are only used to demonstrated the feasibility of doing performance
> profiling in KVM. Feel free to use it if you want.
>    

Performance monitoring is an important feature for kvm.  Is there any 
chance you can work at getting it into good shape?

> We categorize performance profiling in a virtualized environment into
> two types: *guest-wide profiling* and *system-wide profiling*. For
> guest-wide profiling, only the guest is profiled. KVM virtualizes the
> PMU and the user runs a profiler directly in the guest. It requires no
> modifications to the guest OS and the profiler running in the guest.
> For system-wide profiling, both KVM and the guest OS are profiled. The
> results are similar to what XenOprof outputs. In this case, one
> profiler running in the host and one profiler running in the guest.
> Still it requires no modifications to the guest and the profiler
> running in it.
>    

Can your implementation support both simultaneously?

> For guest-wide profiling, there are two possible places to save and
> restore the related MSRs. One is where the CPU switches between guest
> mode and host mode. We call this *CPU-switch*. Profiling with this
> enabled reflects how the guest behaves on the physical CPU, plus other
> virtualized, not emulated, devices. The other place is where the CPU
> switches between the KVM context and others. Here KVM context means
> the CPU is executing guest code or KVM code, both kernel space and
> user space. We call this *domain-switch*. Profiling with this enabled
> discloses how the guest behaves on both the physical CPU and KVM.
> (Some emulated operations are really expensive in a virtualized
> environment.)
>    

Which method do you use?  Or do you support both?

Note disclosing host pmu data to the guest is sometimes a security issue.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.