linux-kernel - Re: [PATCH] x86/i386: make sure stack-protector segment base is cache aligned

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Date:	Thu, 03 Sep 2009 13:41:28 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/i386: make sure stack-protector segment base is cache
 aligned

On 09/03/09 12:47, Eric Dumazet wrote:
> Jeremy Fitzhardinge a écrit :
>   
>> The Intel Optimization Reference Guide says:
>>
>> 	In Intel Atom microarchitecture, the address generation unit
>> 	assumes that the segment base will be 0 by default. Non-zero
>> 	segment base will cause load and store operations to experience
>> 	a delay.
>> 		- If the segment base isn't aligned to a cache line
>> 		  boundary, the max throughput of memory operations is
>> 		  reduced to one [e]very 9 cycles.
>> 	[...]
>> 	Assembly/Compiler Coding Rule 15. (H impact, ML generality)
>> 	For Intel Atom processors, use segments with base set to 0
>> 	whenever possible; avoid non-zero segment base address that is
>> 	not aligned to cache line boundary at all cost.
>>
>> We can't avoid having a non-zero base for the stack-protector segment, but
>> we can make it cache-aligned.
>>
>> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
>>
>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>> index 0bfcf7e..f7d2c8f 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -403,7 +403,17 @@ extern unsigned long kernel_eflags;
>>  extern asmlinkage void ignore_sysret(void);
>>  #else	/* X86_64 */
>>  #ifdef CONFIG_CC_STACKPROTECTOR
>> -DECLARE_PER_CPU(unsigned long, stack_canary);
>> +/*
>> + * Make sure stack canary segment base is cached-aligned:
>> + *   "For Intel Atom processors, avoid non zero segment base address
>> + *    that is not aligned to cache line boundary at all cost."
>> + * (Optim Ref Manual Assembly/Compiler Coding Rule 15.)
>> + */
>> +struct stack_canary {
>> +	char __pad[20];		/* canary at %gs:20 */
>> +	unsigned long canary;
>> +};
>> +DECLARE_PER_CPU(struct stack_canary, stack_canary) ____cacheline_aligned;
>>     
> DECLARE_PER_CPU_SHARED_ALIGNED()
>
> Or else, we'll have many holes in percpu section, because of linker encapsulation
>   

That's only cache aligned when SMP is enabled, to avoid false cacheline
sharing.  In this case we need it unconditionally cache-aligned.

    J
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