Re: [PATCH 1/2] sparc64: Ensure perf can access user stacks

[Rob Gardner <rob.gardner@oracle.com>]

On 12/25/2015 11:27 PM, David Miller wrote:
> From: Rob Gardner <rob.gardner@oracle.com>
> Date: Fri, 25 Dec 2015 22:02:43 -0700
>
>> Also, in the code we submitted, there was an optimization in which
>> %asi is read, and then only set to ASI_AIUS if necessary. This
>> drastically reduces the number of writes to the %asi register since
>> most of the time %asi will contain ASI_AIUS. This seems like a
>> reasonable optimization, since this function may be called thousands
>> of times per second on every cpu.
> I noticed the optimization.
>
> If this was happening for every memcpy call, I'd say it's worth it.
>
> But it's happening once for a series of memcpy/copy_from_user_inatomic()
> calls so I'd say it's not really worth it.
>
> So unless you can show me how the current version fails, I'm keeping it
> as-is because either we should consistently use set_fs/get_fs in C
> code rather than open coded inline asm, or we should create a well
> documented set of helper functions for this specific situation and
> _ALSO_ use it elsewhere where the same problems exist such as some
> of the uses of set_fs/get_fs in process_64.c
>

Fair enough. You've convinced me that my worries are unfounded. Let's 
consider the matter settled. Thanks.

Rob