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Date: Mon, 23 Mar 2015 18:51:11 +0100
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Steven Rostedt <rostedt@goodmis.org>
CC: Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] x86: stop using PER_CPU_VAR(kernel_stack)
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On 03/23/2015 06:45 PM, Steven Rostedt wrote:
> On Mon, 23 Mar 2015 18:38:09 +0100
> Denys Vlasenko <dvlasenk@redhat.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.