linux-kernel - Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER, PR_MPX

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Date:	Fri, 12 Sep 2014 13:35:12 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Qiaowei Ren <qiaowei.ren@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER,
 PR_MPX_UNREGISTER

On 09/12/2014 11:42 AM, Thomas Gleixner wrote:
> On Fri, 12 Sep 2014, Thomas Gleixner wrote:
>> On Fri, 12 Sep 2014, Dave Hansen wrote:
>> The proper solution to this problem is:
>>
>>     do_bounds()
>> 	bd_addr = get_bd_addr_from_xsave();
>> 	bd_entry = bndstatus & ADDR_MASK:
> 
> Just for clarification. You CANNOT avoid the xsave here because it's
> the only way to access BNDSTATUS according to the manual.
> 
> "The BNDCFGU and BNDSTATUS registers are accessible only with
>  XSAVE/XRSTOR family of instructions"
> 
> So there is no point to cache BNDCFGU as you get it anyway when you
> need to retrieve the invalid BD entry.

Agreed.  It serves no purpose during a bounds fault.

However, it does keep you from having to do an xsave during the bounds
table free operations, like at unmap() time.  That is actually a much
more critical path than bounds faults.

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