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

[Peter Zijlstra]

On Fri, Mar 06, 2020 at 01:56:55AM +0800, Luwei Kang wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> The PEBS event created by host needs to be assigned specific counters
> requested by the guest, which means the guest and host counter indexes
> have to be the same or fail to create. This is needed because PEBS leaks
> counter indexes into the guest. Otherwise, the guest driver will be
> confused by the counter indexes in the status field of the PEBS record.
> 
> A guest_dedicated_idx field is added to indicate the counter index
> specifically requested by KVM. The dedicated event constraints would
> constrain the counter in the host to the same numbered counter in guest.
> 
> A intel_ctrl_guest_dedicated_mask field is added to indicate the enabled
> counters for guest PEBS events. The IA32_PEBS_ENABLE MSR will be switched
> during the VMX transitions if intel_ctrl_guest_owned is set.
> 

> +	/* the guest specified counter index of KVM owned event, e.g PEBS */
> +	int				guest_dedicated_idx;

We've always objected to guest 'owned' counters, they destroy scheduling
freedom. Why are you expecting that to be any different this time?