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

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Date:   Fri, 5 May 2017 13:41:24 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Mathias Nyman <mathias.nyman@...ux.intel.com>, tglx@...utronix.de,
        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 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?

Best regards,
Lu Baolu

>
> -boris
>
>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: x86@...nel.org
>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>> ---
>>  arch/x86/kernel/setup.c | 22 ++++++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>>
>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>> index 4bf0c89..e70204e 100644
>> --- a/arch/x86/kernel/setup.c
>> +++ b/arch/x86/kernel/setup.c
>> @@ -837,6 +837,26 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
>>  	return 0;
>>  }
>>  
>> +static void __init simple_udelay_calibration(void)
>> +{
>> +	unsigned int tsc_khz, cpu_khz;
>> +	unsigned long lpj;
>> +
>> +	if (!boot_cpu_has(X86_FEATURE_TSC))
>> +		return;
>> +
>> +	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
>