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Date: Fri, 10 Apr 2015 11:09:37 -0700
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Subject: Re: [PATCH] x86/uaccess: Implement get_kernel()
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Jason Low <jason.low2@hp.com>, Peter Zijlstra <peterz@infradead.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Apr 10, 2015 at 11:04 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
>>
>>     So maybe "get_kernel_stalepointer()" or something like that.
>
> Ok.

Note that I'm not at all married to that particular name, I just want
something that describes the requirements a bit more.

>>  - I think we should look at sharing the code for __get_user(). Could
>> we do something like this:
>>
>>      (a) implement the basic "load with exceptions" as __get_with_exception()
>>      (b) #define get_kernel_stalepointer() __get_with_exception
>>      (c) make "__get_user()" be "stac(); __get_with_exception(); clac()"
>
> Will try.
>
> The only possible complication there might be the way we don't recover
> the error code in the _ex() variants, that's actually a pretty
> important aspect to making this zero cost.

Yeah. You may be right. What I would really want is that "asm goto"
with an output register, but gcc doesn't do that. Then we could
improve on the whole try/catch thing too, so that it would just jump
to the catch..

>>         #ifdef CONFIG_DEBUG_PAGEALLOC
>>             #define get_kernel_stalepointer(x,ptr) ((x)=READ_ONCE(*(ptr)), 0)
>>         #else
>>            #define get_kernel_stalepointer(x,ptr) __get_with_exception(x,ptr)
>>         #endif
>
> I guess you meant that to be the other way around?

Yes I did.

                  Linus