RE: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a dedicated counter for guest PEBS

[Kang, Luwei]

> > On 3/9/2020 11:05 AM, Peter Zijlstra wrote:
> >>> In the new proposal, KVM user is treated the same as other host
> >>> events with event constraint. The scheduler is free to choose
> >>> whether or not to assign a counter for it.
> >> That's what it does, I understand that. I'm saying that that is
> >> creating artificial contention.
> >>
> >>
> >> Why is this needed anyway? Can't we force the guest to flush and then
> >> move it over to a new counter?
> >
> 
> Current perf scheduling is pure software behavior. KVM only traps the MSR
> access. Itâs impossible for KVM to impact the guestâs scheduling with current
> implementation.
> 
> To address the concern regarding to 'artificial contention', we have two
> proposals.
> Could you please take a look, and share your thoughts?
> 
> Proposal 1:
> Reject the guest request, if host has to use the counter which occupied by
> guest. At the meantime, host prints a warning.
> I still think the contention should rarely happen in practical.
> Personally, I prefer this proposal.
> 
> 
> Proposal 2:
> Add HW advisor for the scheduler in guest.
> Starts from Architectural Perfmon Version 4, IA32_PERF_GLOBAL_INUSE MSR
> is introduced. It provides an âInUseâ bit for each programmable
> performance counter and fixed counter in the processor.
> 
> In perf, the scheduler will read the MSR and mask the âin usedâ
> counters. I think we can use X86_FEATURE_HYPERVISOR to limit the check
> in guest. For non-virtualization usage and host, nothing changed for
> scheduler.
> 
> But there is still a problem for this proposal. Host may request a
> counter later, which has been used by guest.
> We can only do multiplexing or grab the counter just like proposal 1.

Hi Peter,
    What is your opinion?

Thanks,
Luwei Kang