linux-kernel - Re: [PATCH v4 09/38] perf: Add switch_guest

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Message-ID: <0355f4e5-e458-457e-ad74-d4dc26331671@linux.intel.com>
Date: Thu, 15 May 2025 09:45:48 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Mingwei Zhang <mizhang@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com,
 Kan <kan.liang@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Yongwei Ma <yongwei.ma@...el.com>,
 Xiong Zhang <xiong.y.zhang@...ux.intel.com>,
 Jim Mattson <jmattson@...gle.com>, Sandipan Das <sandipan.das@....com>,
 Zide Chen <zide.chen@...el.com>, Eranian Stephane <eranian@...gle.com>,
 Shukla Manali <Manali.Shukla@....com>,
 Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 09/38] perf: Add switch_guest_ctx() interface

On 5/15/2025 7:30 AM, Sean Christopherson wrote:
> On Mon, Mar 24, 2025, Mingwei Zhang wrote:
>> From: Kan Liang <kan.liang@...ux.intel.com>
>>
>> When entering/exiting a guest, some contexts for a guest have to be
>> switched. For examples, there is a dedicated interrupt vector for
>> guests on Intel platforms.
>>
>> When PMI switch into a new guest vector, guest_lvtpc value need to be
>> reflected onto HW, e,g., guest clear PMI mask bit, the HW PMI mask
>> bit should be cleared also, then PMI can be generated continuously
>> for guest. So guest_lvtpc parameter is added into perf_guest_enter()
>> and switch_guest_ctx().
>>
>> Add a dedicated list to track all the pmus with the PASSTHROUGH cap, which
>> may require switching the guest context. It can avoid going through the
>> huge pmus list.
>>
>> Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
>> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
>> ---
>>  include/linux/perf_event.h | 17 +++++++++++--
>>  kernel/events/core.c       | 51 +++++++++++++++++++++++++++++++++++++-
>>  2 files changed, 65 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index 37187ee8e226..58c1cf6939bf 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -584,6 +584,11 @@ struct pmu {
>>  	 * Check period value for PERF_EVENT_IOC_PERIOD ioctl.
>>  	 */
>>  	int (*check_period)		(struct perf_event *event, u64 value); /* optional */
>> +
>> +	/*
>> +	 * Switch guest context when a guest enter/exit, e.g., interrupt vectors.
>> +	 */
>> +	void (*switch_guest_ctx)	(bool enter, void *data); /* optional */
> IMO, putting this in "struct pmu" is unnecessarily convoluted and complex, and a
> poor fit for what needs to be done.  The only usage of the hook is for the CPU to
> swap the LVTPC, and the @data payload communicates exactly that.  I.e. this has
> one user, and can't reasonably be extended to other users without some ugliness.
>
> And if by some miracle there's no CPU pmu in perf, KVM's mediated PMU still needs
> to swap to its PMI IRQ.  So rather than per-PMU hook along with a list and a
> spinlock, just make this an arch hook.  And if all of the mediated PMU code is
> guarded by a Kconfig, then perf doesn't even needs __weak stubs.

Sounds good for me.

>