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

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Message-ID: <5966E247.9090700@linux.intel.com>
Date:   Thu, 13 Jul 2017 11:00:23 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Dou Liyang <douly.fnst@...fujitsu.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,

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().

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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