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

From: Keqian Zhu <zhukeqian1@huawei.com>
To: Marc Zyngier <maz@kernel.org>
Cc: <kvm@vger.kernel.org>, <kvmarm@lists.cs.columbia.edu>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, <linux-s390@vger.kernel.org>,
	<linux-sh@vger.kernel.org>, Rich Felker <dalias@libc.org>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	"Peter Zijlstra" <peterz@infradead.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Heiko Carstens <hca@linux.ibm.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>, <nathan@kernel.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	<kernel-team@android.com>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH 1/5] KVM: arm64: Divorce the perf code from oprofile helpers
Date: Thu, 15 Apr 2021 19:34:40 +0800	[thread overview]
Message-ID: <da73e0c7-4dd7-ce69-3304-3da8f1521127@huawei.com> (raw)
In-Reply-To: <87h7k7n81z.wl-maz@kernel.org>

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.

> 
> This of course is broken in other ways (BL+KVM is a total disaster
> when it comes to PMU), but making this static would just make it
> worse.
OK.

Thanks,
Keqian