linux-kernel - Re: [PATCH 2/2] jump labels/x86: Use etiher 5 byte or 2 byte jumps

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Date:	Thu, 19 Jan 2012 06:46:54 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jason Baron <jbaron@...hat.com>
Subject: Re: [PATCH 2/2] jump labels/x86: Use etiher 5 byte or 2 byte jumps

On 01/19/2012 06:41 AM, Mathieu Desnoyers wrote:
>> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
>> index a32b18c..872b3e1 100644
>> --- a/arch/x86/include/asm/jump_label.h
>> +++ b/arch/x86/include/asm/jump_label.h
>> @@ -14,7 +14,7 @@
>>  static __always_inline bool arch_static_branch(struct jump_label_key *key)
>>  {
>>  	asm goto("1:"
>> -		JUMP_LABEL_INITIAL_NOP
>> +		"jmp %l[l_yes]\n"
> 
> Is it possible that the compiler choose a jump that is not 2 or 5-byte ?
> e.g. a jmp rel16 (e9 opcode) on 32-bit x86, or any of the other
> instruction listed under the JMP-Jump instruction in the Intel insn
> manual ?
> 

No.

>>  				   void *(*poker)(void *, const void *, size_t))
>>  {
>>  	union jump_code_union code;
>> +	unsigned char nop;
>> +	unsigned char op;
>> +	unsigned size;
>> +	void *ip = (void *)entry->code;
>> +	void *ideal = (void *)ideal_nops[NOP_ATOMIC5];
> 
> "void *" should possibly be "unsigned char *" here to respect the nop
> place-holder typing.
> 

const unsigned char * please.

>> +
>> +	/* Use probe_kernel_read()? */
>> +	op = *(unsigned char *)ip;
>> +	nop = ideal_nops[NOP_ATOMIC5][0];
>>  
>>  	if (type == JUMP_LABEL_ENABLE) {
>> -		code.jump = 0xe9;
>> -		code.offset = entry->target -
>> -				(entry->code + JUMP_LABEL_NOP_SIZE);
>> -	} else
>> -		memcpy(&code, ideal_nops[NOP_ATOMIC5], JUMP_LABEL_NOP_SIZE);
>> +		if (op == 0xe9 || op == 0xeb)
>> +			/* Already enabled. Warn? */
> 
> This could be caused by failure to run the link-time script, or running
> the transform twice. A warning would indeed be welcome, as this should
> never happen.
> 

Warning?  No.  ERROR.  Something very bad could be happening here.  We
have covered this before.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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