Re: [PATCH 1/5] KVM: arm64: Divorce the perf code from oprofile helpers

From: Marc Zyngier <maz@kernel.org>
To: Keqian Zhu <zhukeqian1@huawei.com>
Cc: linux-s390@vger.kernel.org, Rich Felker <dalias@libc.org>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	kvm@vger.kernel.org, linux-sh@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Heiko Carstens <hca@linux.ibm.com>,
	linux-kernel@vger.kernel.org,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	nathan@kernel.org, Christian Borntraeger <borntraeger@de.ibm.com>,
	Will Deacon <will@kernel.org>,
	kernel-team@android.com, kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/5] KVM: arm64: Divorce the perf code from oprofile helpers
Date: Thu, 15 Apr 2021 14:49:30 +0100	[thread overview]
Message-ID: <87czuvmzdh.wl-maz@kernel.org> (raw)
In-Reply-To: <da73e0c7-4dd7-ce69-3304-3da8f1521127@huawei.com>

On Thu, 15 Apr 2021 12:34:40 +0100,
Keqian Zhu <zhukeqian1@huawei.com> wrote:
> 
> Hi Marc,
> 
> On 2021/4/15 18:42, Marc Zyngier wrote:
> > On Thu, 15 Apr 2021 07:59:26 +0100,
> > Keqian Zhu <zhukeqian1@huawei.com> wrote:
> >>
> >> Hi Marc,
> >>
> >> On 2021/4/14 21:44, Marc Zyngier wrote:
> >>> KVM/arm64 is the sole user of perf_num_counters(), and really
> >>> could do without it. Stop using the obsolete API by relying on
> >>> the existing probing code.
> >>>
> >>> Signed-off-by: Marc Zyngier <maz@kernel.org>
> >>> ---
> >>>  arch/arm64/kvm/perf.c     | 7 +------
> >>>  arch/arm64/kvm/pmu-emul.c | 2 +-
> >>>  include/kvm/arm_pmu.h     | 4 ++++
> >>>  3 files changed, 6 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/arch/arm64/kvm/perf.c b/arch/arm64/kvm/perf.c
> >>> index 739164324afe..b8b398670ef2 100644
> >>> --- a/arch/arm64/kvm/perf.c
> >>> +++ b/arch/arm64/kvm/perf.c
> >>> @@ -50,12 +50,7 @@ static struct perf_guest_info_callbacks kvm_guest_cbs = {
> >>>  
> >>>  int kvm_perf_init(void)
> >>>  {
> >>> -	/*
> >>> -	 * Check if HW_PERF_EVENTS are supported by checking the number of
> >>> -	 * hardware performance counters. This could ensure the presence of
> >>> -	 * a physical PMU and CONFIG_PERF_EVENT is selected.
> >>> -	 */
> >>> -	if (IS_ENABLED(CONFIG_ARM_PMU) && perf_num_counters() > 0)
> >>> +	if (kvm_pmu_probe_pmuver() != 0xf)
> >> The probe() function may be called many times
> >> (kvm_arm_pmu_v3_set_attr also calls it).  I don't know whether the
> >> first calling is enough. If so, can we use a static variable in it,
> >> so the following calling can return the result right away?
> > 
> > No, because that wouldn't help with crappy big-little implementations
> > that could have PMUs with different versions. We want to find the
> > version at the point where the virtual PMU is created, which is why we
> > call the probe function once per vcpu.
> I see.
> 
> But AFAICS the pmuver is placed in kvm->arch, and the probe function is called
> once per VM. Maybe I miss something.

You're right, I mis-remembered. This doesn't change much though.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.
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