Date:	Wed, 12 Aug 2015 11:26:41 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Xen Devel <Xen-devel@...ts.xensource.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, linux-next@...r.kernel.org,
	linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: linux-next: manual merge of the xen-tip tree with the tip tree

One option might be to do the addition in assembly, i.e.:

"i" (key), "i" (index)

... and put the addition into the assembly source.

On August 12, 2015 11:17:17 AM PDT, Boris Ostrovsky <boris.ostrovsky@...cle.com> wrote:
>On 08/12/2015 01:46 PM, Peter Zijlstra wrote:
>> On Wed, Aug 12, 2015 at 07:21:05PM +0200, Peter Zijlstra wrote:
>>> On Wed, Aug 12, 2015 at 09:27:38AM -0400, Boris Ostrovsky wrote:
>>>
>>>> Incidentally, 11276d53 ("locking/static_keys: Add a new static_key
>>>> interface") breaks old-ish compilers (gcc version 4.4.4 20100503
>(Red Hat
>>>> 4.4.4-2) (GCC)):
>>>>
>>>>
>>>>
>>>>    CC      arch/x86/kernel/nmi.o
>>>> In file included from
>>>> /home/build/linux-boris/include/linux/jump_label.h:109,
>>>>                   from
>>>> /home/build/linux-boris/arch/x86/include/asm/spinlock.h:5,
>>>>                   from
>/home/build/linux-boris/include/linux/spinlock.h:88,
>>>>                   from
>/home/build/linux-boris/arch/x86/kernel/nmi.c:14:
>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h: In
>function
>>>> ‘nmi_handle’:
>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21:
>warning: asm
>>>> operand 0 probably doesn’t match constraints
>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21:
>error:
>>>> impossible constraint in ‘asm’
>>>> make[3]: *** [arch/x86/kernel/nmi.o] Error 1
>>>> make[2]: *** [arch/x86/kernel] Error 2
>>>> make[1]: *** [arch/x86] Error 2
>>> Ugh bugger.
>>>
>>> I bet its that: &((char *)key)[branch] business, an earlier variant
>>> thereof tripped up more recent GCCs too.
>>>
>>> So its an __always_inline function, and both argument are always
>compile
>>> time constants, @key is the address of an object in static storage
>(a
>>> global) and @branch is a simple 0/1 at the call site.
>>>
>>> Now we wish to compute (unsigned long)key + branch at compile/link
>time
>>> to feed to the assembler as an immediate, which should be possible,
>>> given its all 'constants'.
>>>
>>> It just appears GCC is having a hard time with this.
>>>
>>> Let me see if I have a sufficiently old GCC around to play with.
>> Could you feed the below to your compiler? Its a bit cumbersome, but
>> its the next best I could come up with...
>
>No, it produces the same error. This is Fedora 13, btw, uses gcc 4.4.4.
>
>-boris
>
>
>>
>> ---
>>   arch/x86/include/asm/jump_label.h | 56
>++++++++++++++++++++++++++++-----------
>>   1 file changed, 41 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/jump_label.h
>b/arch/x86/include/asm/jump_label.h
>> index 28d7a857f9d1..76c769ae4200 100644
>> --- a/arch/x86/include/asm/jump_label.h
>> +++ b/arch/x86/include/asm/jump_label.h
>> @@ -16,15 +16,30 @@
>>   # define STATIC_KEY_INIT_NOP GENERIC_NOP5_ATOMIC
>>   #endif
>>   
>> +struct foo {
>> +	u8 zero;
>> +	u8 one;
>> +};
>> +
>>   static __always_inline bool arch_static_branch(struct static_key
>*key, bool branch)
>>   {
>> -	asm_volatile_goto("1:"
>> -		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
>> -		".pushsection __jump_table,  \"aw\" \n\t"
>> -		_ASM_ALIGN "\n\t"
>> -		_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
>> -		".popsection \n\t"
>> -		: :  "i" (&((char *)key)[branch]) : : l_yes);
>> +	if (!branch) {
>> +		asm_volatile_goto("1:"
>> +				".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
>> +				".pushsection __jump_table,  \"aw\" \n\t"
>> +				_ASM_ALIGN "\n\t"
>> +				_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
>> +				".popsection \n\t"
>> +				: :  "i" (&((struct foo *)key)->zero) : : l_yes);
>> +	} else {
>> +		asm_volatile_goto("1:"
>> +				".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
>> +				".pushsection __jump_table,  \"aw\" \n\t"
>> +				_ASM_ALIGN "\n\t"
>> +				_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
>> +				".popsection \n\t"
>> +				: :  "i" (&((struct foo *)key)->one) : : l_yes);
>> +	}
>>   
>>   	return false;
>>   l_yes:
>> @@ -33,14 +48,25 @@ static __always_inline bool
>arch_static_branch(struct static_key *key, bool bran
>>   
>>   static __always_inline bool arch_static_branch_jump(struct
>static_key *key, bool branch)
>>   {
>> -	asm_volatile_goto("1:"
>> -		".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
>> -		"2:\n\t"
>> -		".pushsection __jump_table,  \"aw\" \n\t"
>> -		_ASM_ALIGN "\n\t"
>> -		_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
>> -		".popsection \n\t"
>> -		: :  "i" (&((char *)key)[branch]) : : l_yes);
>> +	if (!branch) {
>> +		asm_volatile_goto("1:"
>> +				".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
>> +				"2:\n\t"
>> +				".pushsection __jump_table,  \"aw\" \n\t"
>> +				_ASM_ALIGN "\n\t"
>> +				_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
>> +				".popsection \n\t"
>> +				: :  "i" (&((struct foo *)key)->zero) : : l_yes);
>> +	} else {
>> +		asm_volatile_goto("1:"
>> +				".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
>> +				"2:\n\t"
>> +				".pushsection __jump_table,  \"aw\" \n\t"
>> +				_ASM_ALIGN "\n\t"
>> +				_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
>> +				".popsection \n\t"
>> +				: :  "i" (&((struct foo *)key)->one) : : l_yes);
>> +	}
>>   
>>   	return false;
>>   l_yes:

-- 
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