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

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Message-ID: <20200306135317.GD12561@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Mar 2020 14:53:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Luwei Kang <luwei.kang@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, tglx@...utronix.de, bp@...en8.de,
        hpa@...or.com, pbonzini@...hat.com,
        sean.j.christopherson@...el.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        pawan.kumar.gupta@...ux.intel.com, ak@...ux.intel.com,
        thomas.lendacky@....com, fenghua.yu@...el.com,
        kan.liang@...ux.intel.com, like.xu@...ux.intel.com
Subject: Re: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a
 dedicated counter for guest PEBS

On Fri, Mar 06, 2020 at 01:56:55AM +0800, Luwei Kang wrote:
> From: Kan Liang <kan.liang@...ux.intel.com>
> 
> 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?