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Subject: Re: [PATCH v2 1/5] powerpc/mm: Enhance 'slice' for supporting PPC32
To:     Christophe LEROY <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Scott Wood <oss@buserror.net>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <49148d07955d3e5f963cedf9adcfcc37c3e03ef4.1516179904.git.christophe.leroy@c-s.fr>
 <87vafyz265.fsf@linux.vnet.ibm.com>
 <84dc1df4-db2f-be11-c1f3-5dddd1e44983@c-s.fr>
From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Date:   Fri, 19 Jan 2018 14:32:15 +0530
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On 01/19/2018 02:14 PM, Christophe LEROY wrote:
> 
> 
> Le 19/01/2018 à 09:24, Aneesh Kumar K.V a écrit :
>> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>
>>> In preparation for the following patch which will fix an issue on
>>> the 8xx by re-using the 'slices', this patch enhances the
>>> 'slices' implementation to support 32 bits CPUs.
>>>
>>> On PPC32, the address space is limited to 4Gbytes, hence only the low
>>> slices will be used. As of today, the code uses
>>> SLICE_LOW_TOP (0x100000000ul) and compares it with addr to determine
>>> if addr refers to low or high space.
>>> On PPC32, such a (addr < SLICE_LOW_TOP) test is always false because
>>> 0x100000000ul degrades to 0. Therefore, the patch modifies
>>> SLICE_LOW_TOP to (0xfffffffful) and modifies the tests to
>>> (addr <= SLICE_LOW_TOP) which will then always be true on PPC32
>>> as addr has type 'unsigned long' while not modifying the PPC64
>>> behaviour.
>>>
>>> This patch moves "slices" functions prototypes from page64.h to page.h
>>>
>>> The high slices use bitmaps. As bitmap functions are not prepared to
>>> handling bitmaps of size 0, the bitmap_xxx() calls are wrapped into
>>> slice_bitmap_xxx() macros which will take care of the 0 nbits case.
>>>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>>> ---
>>>   v2: First patch of v1 serie split in two parts ; added 
>>> slice_bitmap_xxx() macros.
>>>
>>>   arch/powerpc/include/asm/page.h      | 14 +++++++++
>>>   arch/powerpc/include/asm/page_32.h   | 19 ++++++++++++
>>>   arch/powerpc/include/asm/page_64.h   | 21 ++-----------
>>>   arch/powerpc/mm/hash_utils_64.c      |  2 +-
>>>   arch/powerpc/mm/mmu_context_nohash.c |  7 +++++
>>>   arch/powerpc/mm/slice.c              | 60 
>>> ++++++++++++++++++++++++------------
>>>   6 files changed, 83 insertions(+), 40 deletions(-)
>>>
>>> diff --git a/arch/powerpc/include/asm/page.h 
>>> b/arch/powerpc/include/asm/page.h
>>> index 8da5d4c1cab2..d0384f9db9eb 100644
>>> --- a/arch/powerpc/include/asm/page.h
>>> +++ b/arch/powerpc/include/asm/page.h
>>> @@ -342,6 +342,20 @@ typedef struct page *pgtable_t;
>>>   #endif
>>>   #endif
>>> +#ifdef CONFIG_PPC_MM_SLICES
>>> +struct mm_struct;
>>> +
>>> +unsigned long slice_get_unmapped_area(unsigned long addr, unsigned 
>>> long len,
>>> +                      unsigned long flags, unsigned int psize,
>>> +                      int topdown);
>>> +
>>> +unsigned int get_slice_psize(struct mm_struct *mm, unsigned long addr);
>>> +
>>> +void slice_set_user_psize(struct mm_struct *mm, unsigned int psize);
>>> +void slice_set_range_psize(struct mm_struct *mm, unsigned long start,
>>> +               unsigned long len, unsigned int psize);
>>> +#endif
>>> +
>>
>> Should we do a slice.h ? the way we have other files? and then do
> 
> Yes we could add a slice.h instead of using page.h for that, good idea.
> 
>>
>> arch/powerpc/include/asm/book3s/64/slice.h that will carry
>> #define slice_bitmap_zero(dst, nbits) \
>>     do { if (nbits) bitmap_zero(dst, nbits); } while (0)
>> #define slice_bitmap_set(dst, pos, nbits) \
>> do { if (nbits) bitmap_set(dst, pos, nbits); } while (0)
>> #define slice_bitmap_copy(dst, src, nbits) \
>> do { if (nbits) bitmap_copy(dst, src, nbits); } while (0)
>> #define slice_bitmap_and(dst, src1, src2, nbits) \
>>     ({ (nbits) ? bitmap_and(dst, src1, src2, nbits) : 0; })
>> #define slice_bitmap_or(dst, src1, src2, nbits) \
>>     do { if (nbits) bitmap_or(dst, src1, src2, nbits); } while (0)
>> #define slice_bitmap_andnot(dst, src1, src2, nbits) \
>>     ({ (nbits) ? bitmap_andnot(dst, src1, src2, nbits) : 0; })
>> #define slice_bitmap_equal(src1, src2, nbits) \
>>     ({ (nbits) ? bitmap_equal(src1, src2, nbits) : 1; })
>> #define slice_bitmap_empty(src, nbits) \
>>     ({ (nbits) ? bitmap_empty(src, nbits) : 1; })
>>
>> This without that if(nbits) check and a proper static inline so that we
>> can do type checking.
> 
> Is it really worth duplicating that just for eliminating the 'if 
> (nbits)' in one case ?
> 
> Only in book3s/64 we will be able to eliminate that, for nohash/32 we 
> need to keep the test due to the difference between low and high slices.

the other advantage is we move the SLICE_LOW_SHIFT to the right 
location. IMHO mm subystem is really complex with these really 
overloaded headers. If we can keep it  seperate we should with minimal 
code duplication?
> 
> In any case, as the nbits we use in slice.c is a constant, the test is 
> eliminated at compilation, so I can't see the benefit of making 

-aneesh