linux-kernel - Re: [v4,17/21] powerpc/8xx: set PTE bit 22 off TLBmiss

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Date:	Fri, 07 Nov 2014 09:00:46 +0100
From:	leroy christophe <christophe.leroy@....fr>
To:	Scott Wood <scottwood@...escale.com>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [v4,17/21] powerpc/8xx: set PTE bit 22 off TLBmiss


Le 07/11/2014 04:37, Scott Wood a écrit :
> On Fri, Sep 19, 2014 at 10:36:09AM +0200, LEROY Christophe wrote:
>> No need to re-set this bit at each TLB miss. Let's set it in the PTE.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
>> Changes in v2:
>> - None
>>
>> Changes in v3:
>> - Removed PPC405 related macro from PPC8xx specific code
>> - PTE_NONE_MASK doesn't need PAGE_ACCESSED in Linux 2.6
>>
>> Changes in v4:
>> - None
>>
>>   arch/powerpc/include/asm/pgtable-ppc32.h | 20 ++++++++++++++++++++
>>   arch/powerpc/include/asm/pte-8xx.h       |  7 +++++--
>>   arch/powerpc/kernel/head_8xx.S           | 10 ++--------
>>   3 files changed, 27 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/pgtable-ppc32.h b/arch/powerpc/include/asm/pgtable-ppc32.h
>> index 47edde8..35a9b44 100644
>> --- a/arch/powerpc/include/asm/pgtable-ppc32.h
>> +++ b/arch/powerpc/include/asm/pgtable-ppc32.h
>> @@ -172,6 +172,25 @@ static inline unsigned long pte_update(pte_t *p,
>>   #ifdef PTE_ATOMIC_UPDATES
>>   	unsigned long old, tmp;
>>   
>> +#ifdef CONFIG_PPC_8xx
>> +	unsigned long tmp2;
>> +
>> +	__asm__ __volatile__("\
>> +1:	lwarx	%0,0,%4\n\
>> +	andc	%1,%0,%5\n\
>> +	or	%1,%1,%6\n\
>> +	/* 0x200 == Extended encoding, bit 22 */ \
>> +	/* Bit 22 has to be 1 if neither _PAGE_USER nor _PAGE_RW are set */ \
>> +	rlwimi	%1,%1,32-2,0x200\n /* get _PAGE_USER */ \
>> +	rlwinm	%3,%1,32-1,0x200\n /* get _PAGE_RW */ \
>> +	or	%1,%3,%1\n\
>> +	xori	%1,%1,0x200\n"
>> +"	stwcx.	%1,0,%4\n\
>> +	bne-	1b"
> Why do you need this...
>
>> diff --git a/arch/powerpc/include/asm/pte-8xx.h b/arch/powerpc/include/asm/pte-8xx.h
>> index d44826e..daa4616 100644
>> --- a/arch/powerpc/include/asm/pte-8xx.h
>> +++ b/arch/powerpc/include/asm/pte-8xx.h
>> @@ -48,19 +48,22 @@
>>    */
>>   #define _PAGE_RW	0x0400	/* lsb PP bits, inverted in HW */
>>   #define _PAGE_USER	0x0800	/* msb PP bits */
>> +/* set when neither _PAGE_USER nor _PAGE_RW are set */
>> +#define _PAGE_KNLRO	0x0200
>>   
>>   #define _PMD_PRESENT	0x0001
>>   #define _PMD_BAD	0x0ff0
>>   #define _PMD_PAGE_MASK	0x000c
>>   #define _PMD_PAGE_8M	0x000c
>>   
>> -#define _PTE_NONE_MASK _PAGE_ACCESSED
>> +#define _PTE_NONE_MASK _PAGE_KNLRO
>>   
>>   /* Until my rework is finished, 8xx still needs atomic PTE updates */
>>   #define PTE_ATOMIC_UPDATES	1
>>   
>>   /* We need to add _PAGE_SHARED to kernel pages */
>> -#define _PAGE_KERNEL_RO	(_PAGE_SHARED)
>> +#define _PAGE_KERNEL_RO	(_PAGE_SHARED | _PAGE_KNLRO)
>> +#define _PAGE_KERNEL_ROX	(_PAGE_EXEC | _PAGE_KNLRO)
>>   #define _PAGE_KERNEL_RW	(_PAGE_DIRTY | _PAGE_RW | _PAGE_HWWRITE)
> ...if 0x200 is already being set in the PTE here?
>
If I understand well the way it works, those defines are used for 
setting the PTE the first time.
Then, pte_update() is used to modify the pte settings on an existing pte

0x200 must be set when and only when the page is a RO kernel page. If 
later on pte_update() is called for instance to set the page to RW, the 
0x200 has to be removed. Same, if pte_update() is called to remove 
_PAGE_RW (ptep_set_wrprotect() does this), 0x200 must be set back.

Christophe

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