linux-kernel - Re: [PATCH] x86: fix ordering constraints on crX read/writes

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Date:	Wed, 14 Jul 2010 17:55:40 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	Glauber Costa <glommer@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Avi Kivity <avi@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: fix ordering constraints on crX read/writes

On 07/14/2010 05:28 PM, Zachary Amsden wrote:
>
>>   static inline void native_write_cr2(unsigned long val)
>>   {
>> -    asm volatile("mov %0,%%cr2": : "r" (val), "m" (__force_order));
>> +    asm volatile("mov %1,%%cr2": "+m" (__force_order) : "r" (val) :
>> "memory");
>>   }
>>    
>
>
> You don't need the memory clobber there.  Technically, this should
> never be used, however.

Yes.  I just did it for consistency.  Likewise, I didn't pore over the
manuals to work out whether writes to any crX could really have memory
side-effects.

>>
>>   static inline unsigned long native_read_cr3(void)
>>   {
>>       unsigned long val;
>> -    asm volatile("mov %%cr3,%0\n\t" : "=r" (val), "=m"
>> (__force_order));
>> +    asm volatile("mov %%cr3,%0\n\t" : "=r" (val) : "m"
>> (__force_order));
>>       return val;
>>   }
>>
>>   static inline void native_write_cr3(unsigned long val)
>>   {
>> -    asm volatile("mov %0,%%cr3": : "r" (val), "m" (__force_order));
>> +    asm volatile("mov %1,%%cr3": "+m" (__force_order) : "r" (val) :
>> "memory");
>>   }
>>
>>   static inline unsigned long native_read_cr4(void)
>>   {
>>       unsigned long val;
>> -    asm volatile("mov %%cr4,%0\n\t" : "=r" (val), "=m"
>> (__force_order));
>> +    asm volatile("mov %%cr4,%0\n\t" : "=r" (val) : "m"
>> (__force_order));
>>       return val;
>>   }
>>
>> @@ -271,7 +286,7 @@ static inline unsigned long
>> native_read_cr4_safe(void)
>>       asm volatile("1: mov %%cr4, %0\n"
>>                "2:\n"
>>                _ASM_EXTABLE(1b, 2b)
>> -             : "=r" (val), "=m" (__force_order) : "0" (0));
>> +             : "=r" (val) : "m" (__force_order), "0" (0));
>>   #else
>>       val = native_read_cr4();
>>   #endif
>> @@ -280,7 +295,7 @@ static inline unsigned long
>> native_read_cr4_safe(void)
>>
>>   static inline void native_write_cr4(unsigned long val)
>>   {
>> -    asm volatile("mov %0,%%cr4": : "r" (val), "m" (__force_order));
>> +    asm volatile("mov %1,%%cr4": "+m" (__force_order) : "r" (val) :
>> "memory");
>>   }
>>
>>   #ifdef CONFIG_X86_64
>>
>>
>>    
>
> Looks good.  I really hope __force_order gets pruned however.  Does it
> actually?

There's a couple of instances in my vmlinux.  I didn't try to track them
back to specific .o files.  gcc tends to generate references by putting
its address into a register and passing that into the asms.

    J

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