From: Marc Zyngier <maz@kernel.org>
To: Paolo Bonzini <pbonzini@redhat.com>,
"Jianyong Wu (Arm Technology China)" <Jianyong.Wu@arm.com>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"yangbo.lu@nxp.com" <yangbo.lu@nxp.com>,
"john.stultz@linaro.org" <john.stultz@linaro.org>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"sean.j.christopherson@intel.com"
<sean.j.christopherson@intel.com>,
"richardcochran@gmail.com" <richardcochran@gmail.com>,
Mark Rutland <Mark.Rutland@arm.com>,
Will Deacon <Will.Deacon@arm.com>,
Suzuki Poulose <Suzuki.Poulose@arm.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
Steve Capper <Steve.Capper@arm.com>,
"Kaly Xin (Arm Technology China)" <Kaly.Xin@arm.com>,
"Justin He (Arm Technology China)" <Justin.He@arm.com>,
nd <nd@arm.com>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH v3 4/6] psci: Add hvc call service for ptp_kvm.
Date: Thu, 19 Sep 2019 12:39:06 +0100 [thread overview]
Message-ID: <a1b554b8-4417-5305-3419-fe71a8c50842@kernel.org> (raw)
In-Reply-To: <ef6ab8bd-41ad-88f8-9cfd-dc749ca65310@redhat.com>
On 19/09/2019 12:07, Paolo Bonzini wrote:
> On 19/09/19 11:46, Jianyong Wu (Arm Technology China) wrote:
>>> On 18/09/19 11:57, Jianyong Wu (Arm Technology China) wrote:
>>>> Paolo Bonzini wrote:
>>>>> This is not Y2038-safe. Please use ktime_get_real_ts64 instead, and
>>>>> split the 64-bit seconds value between val[0] and val[1].
>>
>> Val[] should be long not u32 I think, so in arm64 I can avoid that Y2038_safe, but
>> also need rewrite for arm32.
>
> I don't think there's anything inherently wrong with u32 val[], and as
> you notice it lets you reuse code between arm and arm64. It's up to you
> and Marc to decide.
>
>>>>> However, it seems to me that the new function is not needed and you
>>>>> can just use ktime_get_snapshot. You'll get the time in
>>>>> systime_snapshot->real and the cycles value in systime_snapshot->cycles.
>>>>
>>>> See patch 5/6, I need both counter cycle and clocksource,
>>> ktime_get_snapshot seems only offer cycles.
>>>
>>> No, patch 5/6 only needs the current clock (ptp_sc.cycles is never accessed).
>>> So you could just use READ_ONCE(tk->tkr_mono.clock).
>>>
>> Yeah, patch 5/6 just need clocksource, but I think tk->tkr_mono.clock can't read in external like module,
>> So I need an API to expose clocksource.
>>
>>> However, even then I don't think it is correct to use ptp_sc.cs blindly in patch
>>> 5. I think there is a misunderstanding on the meaning of
>>> system_counterval.cs as passed to get_device_system_crosststamp.
>>> system_counterval.cs is not the active clocksource; it's the clocksource on
>>> which system_counterval.cycles is based.
>>>
>>
>> I think we can use system_counterval_t as pass current clocksource to system_counterval_t.cs and its
>> corresponding cycles to system_counterval_t.cycles. is it a big problem?
>
> Yes, it is. Because...
>
>>> Hypothetically, the clocksource could be one for which ptp_sc.cycles is _not_
>>> a cycle value. If you set system_counterval.cs to the system clocksource,
>>> get_device_system_crosststamp will return a bogus value.
>>
>> Yeah, but in patch 3/6, we have a corresponding pair of clock source and cycle value. So I think there will be no
>> that problem in this patch set.
>> In the implementation of get_device_system_crosststamp:
>> "
>> ...
>> if (tk->tkr_mono.clock != system_counterval.cs)
>> return -ENODEV;
>> ...
>> "
>> We need tk->tkr_mono.clock passed to get_device_system_crosststamp, just like patch 3/6 do, otherwise will return error.
>
> ... if the hypercall returns an architectural timer value, you must not
> pass tk->tkr.mono.clock to get_device_system_crosststamp: you must pass
> &clocksource_counter. This way, PTP is disabled when using any other
> clocksource.
>
>>> So system_counterval.cs should be set to something like
>>> &clocksource_counter (from drivers/clocksource/arm_arch_timer.c).
>>> Perhaps the right place to define kvm_arch_ptp_get_clock_fn is in that file?
>>>
>> I have checked that ptp_sc.cs is arch_sys_counter.
>> Also move the module API to arm_arch_timer.c will looks a little
>> ugly and it's not easy to be accept by arm side I think.
>
> I don't think it's ugly but more important, using tk->tkr_mono.clock is
> incorrect. See how the x86 code hardcodes &kvm_clock, it's the same for
> ARM.
Not really. The guest kernel is free to use any clocksource it wishes.
In some cases, it is actually desirable (like these broken systems that
cannot use an in-kernel irqchip...). Maybe it is that on x86 the guest
only uses the kvm_clock, but that's a much harder sell on ARM. The fact
that ptp_kvm assumes that the clocksource is fixed doesn't seem correct
in that case.
M.
--
Jazz is not dead, it just smells funny...
next prev parent reply other threads:[~2019-09-19 11:39 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-18 8:07 [RFC PATCH v3 0/6] Enable ptp_kvm for arm64 Jianyong Wu
2019-09-18 8:07 ` [RFC PATCH v3 1/6] psci: Export psci_ops.conduit symbol as modules will use it Jianyong Wu
2019-09-18 8:07 ` [RFC PATCH v3 2/6] ptp: Reorganize ptp_kvm modules to make it arch-independent Jianyong Wu
2019-09-18 8:07 ` [RFC PATCH v3 3/6] timekeeping: Expose API allowing retrival of current clocksource and counter value Jianyong Wu
2019-09-18 8:29 ` Paolo Bonzini
2019-09-18 8:07 ` [RFC PATCH v3 4/6] psci: Add hvc call service for ptp_kvm Jianyong Wu
2019-09-18 8:25 ` Paolo Bonzini
2019-09-18 9:57 ` Jianyong Wu (Arm Technology China)
2019-09-18 10:23 ` Paolo Bonzini
2019-09-19 9:46 ` Jianyong Wu (Arm Technology China)
2019-09-19 11:07 ` Paolo Bonzini
2019-09-19 11:39 ` Marc Zyngier [this message]
2019-09-19 12:13 ` Paolo Bonzini
2019-09-23 3:19 ` Jianyong Wu (Arm Technology China)
2019-10-09 5:21 ` Jianyong Wu (Arm Technology China)
2019-10-09 6:36 ` Paolo Bonzini
2019-10-09 8:18 ` Jianyong Wu (Arm Technology China)
2019-10-09 9:13 ` Paolo Bonzini
2019-10-09 16:05 ` John Stultz
2019-10-09 20:56 ` Paolo Bonzini
2019-10-14 5:50 ` Jianyong Wu (Arm Technology China)
2019-10-14 6:58 ` Paolo Bonzini
2019-09-23 4:57 ` Jianyong Wu (Arm Technology China)
2019-09-24 14:20 ` Paolo Bonzini
2019-09-25 10:27 ` Jianyong Wu (Arm Technology China)
2019-09-18 8:07 ` [RFC PATCH v3 5/6] ptp: arm64: Enable ptp_kvm for arm64 Jianyong Wu
2019-09-18 8:07 ` [RFC PATCH v3 6/6] kvm: arm64: Add capability check extension for ptp_kvm Jianyong Wu
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