linux-kernel - Re: [PATCH 2/7] x86, xsave: introduce xstate enable functions

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Date:	Thu, 22 Jul 2010 14:15:22 +0200
From:	Robert Richter <robert.richter@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/7] x86, xsave: introduce xstate enable functions

On 21.07.10 17:53:31, H. Peter Anvin wrote:

> From 55b936c7a359a14d72bcba6c3fceba4cfbe3fedf Mon Sep 17 00:00:00 2001
> From: H. Peter Anvin <hpa@...ux.intel.com>
> Date: Wed, 21 Jul 2010 14:23:10 -0700
> Subject: [PATCH] x86, xsave: Make xstate_enable_boot_cpu() __init, protect on CPU 0
> 
> xstate_enable_boot_cpu() is, as the name implies, only used on the
> boot CPU; furthermore, it invokes alloc_bootmem(), which is __init;
> hence it needs to be tagged __init rather than __cpuinit.
> 
> Furthermore, it is *not* safe in the long run to rely on CPU 0 only
> coming online during the early boot -- at some point we're going to
> support offlining (and re-onlining) the boot CPU, and at that point we
> must not call xstate_enable_boot_cpu() again.
> 
> The code is a fair bit more obscure than one would like, because the
> __ref overrides aren't quite powerful enough.
> 
> Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
> Acked-by: Suresh Siddha <suresh.b.siddha@...el.com>
> Cc: Robert Richter <robert.richter@....com>
> LKML-Reference: <4C476236.1020302@...or.com>

I am fine with your changes.

>  void __cpuinit xsave_init(void)
>  {
> +	static __refdata void (*next_func)(void) = xstate_enable_boot_cpu;
> +	void (*this_func)(void);
> +
>  	if (!cpu_has_xsave)
>  		return;
>  
> -	/*
> -	 * Boot processor to setup the FP and extended state context info.
> -	 */
> -	if (!smp_processor_id())
> -		xstate_enable_boot_cpu();
> -	else
> -		xstate_enable(pcntxt_mask);
> +	this_func = next_func;
> +	next_func = xstate_enable;
> +	this_func();
>  }

Just wondering why you are using this_func()? Instead, you could
simply do:

	next_func();
	next_func = xstate_enable;

Do you see races when bringing up multiple cpus in parallel?

Thanks.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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