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From: "Liang, Kan" <kan.liang@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Luwei Kang <luwei.kang@intel.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, mingo@redhat.com, acme@kernel.org,
	mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
	jolsa@redhat.com, namhyung@kernel.org, tglx@linutronix.de,
	bp@alien8.de, hpa@zytor.com, pbonzini@redhat.com,
	sean.j.christopherson@intel.com, vkuznets@redhat.com,
	wanpengli@tencent.com, jmattson@google.com, joro@8bytes.org,
	pawan.kumar.gupta@linux.intel.com, ak@linux.intel.com,
	thomas.lendacky@amd.com, fenghua.yu@intel.com,
	like.xu@linux.intel.com
Subject: Re: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a dedicated counter for guest PEBS
Date: Mon, 9 Mar 2020 15:28:31 -0400	[thread overview]
Message-ID: <45a1a575-9363-f778-b5f5-bcdf28d3e34b@linux.intel.com> (raw)
In-Reply-To: <20200309150526.GI12561@hirez.programming.kicks-ass.net>



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

  reply	other threads:[~2020-03-09 19:28 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-05 17:56 [PATCH v1 00/11] PEBS virtualization enabling via DS Luwei Kang
2020-03-05 16:51 ` Paolo Bonzini
2020-03-05 17:56 ` [PATCH v1 01/11] perf/x86/core: Support KVM to assign a dedicated counter for guest PEBS Luwei Kang
2020-03-06 13:53   ` Peter Zijlstra
2020-03-06 14:42     ` Liang, Kan
2020-03-09 10:04       ` Peter Zijlstra
2020-03-09 13:12         ` Liang, Kan
2020-03-09 15:05           ` Peter Zijlstra
2020-03-09 19:28             ` Liang, Kan [this message]
2020-03-12 10:28               ` Kang, Luwei
2020-03-26 14:03               ` Liang, Kan
2020-04-07 12:34                 ` Kang, Luwei
2020-06-12  5:28             ` Kang, Luwei
2020-06-19  9:30               ` Kang, Luwei
2020-08-20  3:32               ` Like Xu
2020-03-09 15:44         ` Andi Kleen
2020-03-05 17:56 ` [PATCH v1 02/11] perf/x86/ds: Handle guest PEBS events overflow and inject fake PMI Luwei Kang
2020-03-05 17:56 ` [PATCH v1 03/11] perf/x86: Expose a function to disable auto-reload Luwei Kang
2020-03-05 17:56 ` [PATCH v1 04/11] KVM: x86/pmu: Decouple event enablement from event creation Luwei Kang
2020-03-05 17:56 ` [PATCH v1 05/11] KVM: x86/pmu: Add support to reprogram PEBS event for guest counters Luwei Kang
2020-03-06 16:28   ` kbuild test robot
2020-03-09  0:58     ` Xu, Like
2020-03-05 17:57 ` [PATCH v1 06/11] KVM: x86/pmu: Implement is_pebs_via_ds_supported pmu ops Luwei Kang
2020-03-05 17:57 ` [PATCH v1 07/11] KVM: x86/pmu: Expose CPUIDs feature bits PDCM, DS, DTES64 Luwei Kang
2020-03-05 17:57 ` [PATCH v1 08/11] KVM: x86/pmu: PEBS MSRs emulation Luwei Kang
2020-03-05 17:57 ` [PATCH v1 09/11] KVM: x86/pmu: Expose PEBS feature to guest Luwei Kang
2020-03-05 17:57 ` [PATCH v1 10/11] KVM: x86/pmu: Introduce the mask value for fixed counter Luwei Kang
2020-03-05 17:57 ` [PATCH v1 11/11] KVM: x86/pmu: Adaptive PEBS virtualization enabling Luwei Kang
2020-03-05 22:48 ` [PATCH v1 00/11] PEBS virtualization enabling via DS Andi Kleen
2020-03-06  5:37   ` Kang, Luwei

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