linux-kernel - Re: [PATCH v8 1/5] x86: add simple udelay calibration

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Date:   Fri, 14 Jul 2017 11:21:14 +0800
From:   Dou Liyang <douly.fnst@...fujitsu.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        <tglx@...utronix.de>
CC:     <peterz@...radead.org>, <linux-usb@...r.kernel.org>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        Juergen Gross <jgross@...e.com>,
        xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH v8 1/5] x86: add simple udelay calibration

Hi, Lu

At 07/13/2017 11:00 AM, Lu Baolu wrote:
> Hi,
>
> On 07/13/2017 09:39 AM, Dou Liyang wrote:
>> Hi, Lu
>>
>> At 07/13/2017 09:17 AM, Lu Baolu wrote:
>>> Hi,
>>>
>>> On 07/12/2017 04:02 PM, Dou Liyang wrote:
>>>> Hi, Lu
>>>>
>>>> At 05/05/2017 08:50 PM, Boris Ostrovsky wrote:
>>>>> On 05/05/2017 01:41 AM, Lu Baolu wrote:
>>>>>> Hi,
>>>>>>
>>>>>> On 05/03/2017 06:38 AM, Boris Ostrovsky wrote:
>>>>>>> On 03/21/2017 04:01 AM, Lu Baolu wrote:
>>>>>>>> Add a simple udelay calibration in x86 architecture-specific
>>>>>>>> boot-time initializations. This will get a workable estimate
>>>>>>>> for loops_per_jiffy. Hence, udelay() could be used after this
>>>>>>>> initialization.
>>>>>>> This breaks Xen PV guests since at this point, and until
>>>>>>> x86_init.paging.pagetable_init() which is when pvclock_vcpu_time_info is
>>>>>>> mapped, they cannot access pvclock.
>>>>>>>
>>>>>>> Is it reasonable to do this before tsc_init() is called? (The failure
>>>>>>> has nothing to do with tsc_init(), really --- it's just that it is
>>>>>>> called late enough that Xen PV guests get properly initialized.) If it
>>>>>>> is, would it be possible to move simple_udelay_calibration() after
>>>>>>> x86_init.paging.pagetable_init()?
>>>>>> This is currently only used for bare metal. How about by-pass it
>>>>>> for Xen PV guests?
>>>>>
>>>>> It is fixed this for Xen PV guests now (in the sense that we don't crash
>>>>> anymore) but my question is still whether this is not too early. Besides
>>>>> tsc_init() (which might not be important here), at the time when
>>>>> simple_udelay_calibration() is invoked we haven't yet called:
>>>>> * kvmclock_init(), which sets calibration routines for KVM
>>>>> * init_hypervisor_platform(), which sets calibration routines for vmware
>>>>> and Xen HVM
>>>>> * x86_init.paging.pagetable_init(), which sets calibration routines for
>>>>> Xen PV
>>>>>
>>>>
>>>> I guess these may have been missed.
>>>>
>>>> Do you have any comments about these?
>>>>
>>>
>>> The patch will be available in 4.13-rc1.
>>
>> Yes, I have seen it in the upstream.
>>
>> Firstly, I also met this problem want to call udelay() earlier than
>> *loops_per_jiffy* setup like you[1]. So I am very interesting in this
>> patch. ;)
>>
>> I am also confused about the questions which Boris asked:
>>
>> whether do the CPU and TSC calibration too early just for using
>> udelay()?
>>
>> this design broke our interface of x86_paltform.calibrate_cpu/tsc.
>>
>> And I also have a question below.
>>
>> [...]
>>
>>>>>>>>
>>>>>>>> +static void __init simple_udelay_calibration(void)
>>>>>>>> +{
>>>>>>>> +    unsigned int tsc_khz, cpu_khz;
>>>>>>>> +    unsigned long lpj;
>>>>>>>> +
>>>>>>>> +    if (!boot_cpu_has(X86_FEATURE_TSC))
>>>>>>>> +        return;
>>
>> if we don't have the TSC feature in booting CPU and
>> it returns here,  can we use udelay() correctly like before?
>>
>
> If we have TSC feature, we calculate a preciser loops_per_jiffy here.
> Otherwise, we just keep it as before. This function doesn't broke the
> use of udelay().

Oh, I see.

In  XDbC (XHCI debug capability), we just want the udelay() work more 
precisely in the TSC supported system.

It is different with my problem I missed.

Thanks for your kind explanation. :)

Thanks,

	dou

>
> Best regards,
> Lu Baolu
>
>>
>> [1] https://lkml.org/lkml/2017/7/3/276
>>
>> Thanks,
>>
>>     dou.
>>
>>>> Thanks,
>>>>
>>>>     dou.
>>>>
>>>>>>>> +
>>>>>>>> +    cpu_khz = x86_platform.calibrate_cpu();
>>>>>>>> +    tsc_khz = x86_platform.calibrate_tsc();
>>>>>>>> +
>>>>>>>> +    tsc_khz = tsc_khz ? : cpu_khz;
>>>>>>>> +    if (!tsc_khz)
>>>>>>>> +        return;
>>>>>>>> +
>>>>>>>> +    lpj = tsc_khz * 1000;
>>>>>>>> +    do_div(lpj, HZ);
>>>>>>>> +    loops_per_jiffy = lpj;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>>  /*
>>>>>>>>   * Determine if we were loaded by an EFI loader.  If so, then we have also been
>>>>>>>>   * passed the efi memmap, systab, etc., so we should use these data structures
>>>>>>>> @@ -985,6 +1005,8 @@ void __init setup_arch(char **cmdline_p)
>>>>>>>>       */
>>>>>>>>      x86_configure_nx();
>>>>>>>>
>>>>>>>> +    simple_udelay_calibration();
>>>>>>>> +
>>>>>>>>      parse_early_param();
>>>>>>>>
>>>>>>>>  #ifdef CONFIG_MEMORY_HOTPLUG
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>> --
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>>>>
>>>
>>>
>>>
>>>
>>
>>
>> --
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>
>
>
>