linux-kernel - Re: [PATCH 03/19] x86, mpx: Use new get_xsave_field

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Date:	Tue, 19 May 2015 10:16:12 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Hansen <dave@...1.net>
cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	dave.hansen@...ux.intel.com, oleg@...hat.com, bp@...en8.de,
	riel@...hat.com, sbsiddha@...il.com, luto@...capital.net,
	mingo@...hat.com, hpa@...or.com, fenghua.yu@...el.com
Subject: Re: [PATCH 03/19] x86, mpx: Use new get_xsave_field_ptr()

On Mon, 18 May 2015, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> The MPX registers (bndcsr/bndcfgu/bndstatus) are not directly
> accessible via normal instructions.  They essentially act as
> if they were floating point registers and are saved/restored
> along with those registers.
> 
> There are two main paths in the MPX code where we care about
> the contents of these registers:
> 	1. #BR (bounds) faults
> 	2. the prctl() code where we are setting MPX up
> 
> Both of those paths _might_ be called without the FPU having
> been used.  That means that 'tsk->thread.fpu.state' might
> never be allocated.
> 
> Also, fpu_save_init() is not preempt-safe.  It was a bug to
> call it without disabling preemption.  The new
> get_xsave_addr() calls unlazy_fpu() instead and properly
> disables preemption.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
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