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

[Liang, Kan]

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?

KVM only traps the MSR access. There is no MSR access during the 
scheduling in guest.
KVM/host only knows the request counter, when guest tries to enable the 
counter. It's too late for guest to start over.

Regarding to the artificial contention, as my understanding, it should 
rarely happen in practical.
Cloud vendors have to explicitly set pebs option in qemu to enable PEBS 
support for guest. They knows the environment well. They can avoid the 
contention. (We may implement some patches for qemu/KVM later to 
temporarily disable PEBS in runtime if they require.)

For now, I think we may print a warning when both host and guest require 
the same counter. Host can get a clue from the warning.

Thanks,
Kan