Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER, PR_MPX_UNREGISTER

[Dave Hansen]

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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