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

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Date:   Thu, 13 Jul 2017 09:39:38 +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 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?


[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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>>
>
>
>
>