linux-kernel - Re: [PATCH 1/2] x86: stop using PER_CPU_VAR(kernel

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Date:	Mon, 23 Mar 2015 11:27:35 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86: stop using PER_CPU_VAR(kernel_stack)

On Mon, Mar 23, 2015 at 10:51 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> On 03/23/2015 06:45 PM, Steven Rostedt wrote:
>> On Mon, 23 Mar 2015 18:38:09 +0100
>> Denys Vlasenko <dvlasenk@...hat.com> wrote:
>>
>>>> Can we do a:
>>>>
>>>>  #define cpu_current_top_of_stack (cpu_tss + TSS_sp0)
>>>
>>> We already do something similar:
>>>
>>> static inline unsigned long current_top_of_stack(void)
>>> {
>>> #ifdef CONFIG_X86_64
>>>         return this_cpu_read_stable(cpu_tss.x86_tss.sp0);
>>> #else
>>>         /* sp0 on x86_32 is special in and around vm86 mode. */
>>>         return this_cpu_read_stable(cpu_current_top_of_stack);
>>> #endif
>>> }
>>
>> So can we then have:
>>
>> #ifdef __ASSEMBLY__
>> # define cpu_current_top_of_stack (cpu_tss + TSS_sp0)
>> #else
>> # define cpu_current_top_of_stack (cpu_tss.x86_tss.sp0)
>> #endif
>>
>> And get rid of that if statement in the static inline?
>
> I prefer less macro indirection in assembly code,
> but I won't object too strongly if this would be done.
>
> It's up to other x86 maintainers to agree - I'm touching
> the code recently changed by Andy, he may see the way forward
> somewhat differently.
>

I kind of like Steven's idea.  Although.. shouldn't we go all the way and do:

#ifdef __ASSEMBLY__
#ifdef CONFIG_X86_64
#define cpu_current_top_of_stack %gs:(cpu_tss + TSS_sp0)
#else
#define cpu_current_top_of_stack %gs:(current_top_of_stack)
#endif
#endif

and just not have a macro for non-asm, since we already have the inline?

But maybe keeping the %gs reference explicit in the asm is better.

--Andy
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