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

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Date:	Fri, 24 Apr 2015 17:29:42 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
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 03/23/2015 07:27 PM, Andy Lutomirski wrote:
> 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

Which .h file do you propose to have this in? processor.h is not suitable,
it is not __ASSEMBLY__-fied.

I'm looking at a place where to put it.
For example, one of the users of the (cpu_tss + TSS_sp0) expression,
xen-asm_64.S, has only these few includes:

#include <asm/errno.h>
#include <asm/percpu.h>
#include <asm/processor-flags.h>
#include <asm/segment.h>
#include <asm/asm-offsets.h>

#include <xen/interface/xen.h>

#include "xen-asm.h"

None seems suitable. I will probably have to include thread_info.h too,
and put the define there. Ugly ugly ugly.

I'll resend a patchset where a new patch #3 attempts to do this
"unification". I personally don't see it as a clear improvement.

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