Date:   Fri, 19 Jan 2018 10:45:35 +0100
From:   Christophe LEROY <christophe.leroy@....fr>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] powerpc/mm: Enhance 'slice' for supporting PPC32



Le 19/01/2018 à 10:13, Aneesh Kumar K.V a écrit :
> 
> 
> On 01/19/2018 02:37 PM, Christophe LEROY wrote:
>>
>>
>> Le 19/01/2018 à 10:02, Aneesh Kumar K.V a écrit :
>>>
>>>
>>> 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@....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@....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?
>>
>> For the constants I fully agree with your proposal and I will do it. I 
>> was only questionning the benefit of moving the slice_bitmap_xxxx() 
>> stuff, taking into account that the 'if (nbits)' test is already 
>> eliminated by the compiler.
>>
> 
> That is compiler dependent as you are finding with the other patch where 
> if (0) didn't get compiled out

I don't think so. When I had the missing prototype, the compilation goes 
ok, including the final link. Which means at the end the code is not 
included since radix_enabled() evaluates to 0.

Many many parts of the kernel are based on this assumption.

Christophe

> 
> -aneesh

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