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

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Date:   Tue, 2 May 2017 18:38:39 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Lu Baolu <baolu.lu@...ux.intel.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

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()?

-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