Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 3/1/22 1:46 PM, Christophe Leroy wrote:
> 
> 
> Le 01/03/2022 à 01:31, Russell King (Oracle) a écrit :
>> On Tue, Mar 01, 2022 at 05:30:41AM +0530, Anshuman Khandual wrote:
>>> On 2/28/22 4:27 PM, Russell King (Oracle) wrote:
>>>> On Mon, Feb 28, 2022 at 04:17:32PM +0530, Anshuman Khandual wrote:
>>>>> This defines and exports a platform specific custom vm_get_page_prot() via
>>>>> subscribing ARCH_HAS_VM_GET_PAGE_PROT. Subsequently all __SXXX and __PXXX
>>>>> macros can be dropped which are no longer needed.
>>>>
>>>> What I would really like to know is why having to run _code_ to work out
>>>> what the page protections need to be is better than looking it up in a
>>>> table.
>>>>
>>>> Not only is this more expensive in terms of CPU cycles, it also brings
>>>> additional code size with it.
>>>>
>>>> I'm struggling to see what the benefit is.
>>>
>>> Currently vm_get_page_prot() is also being _run_ to fetch required page
>>> protection values. Although that is being run in the core MM and from a
>>> platform perspective __SXXX, __PXXX are just being exported for a table.
>>> Looking it up in a table (and applying more constructs there after) is
>>> not much different than a clean switch case statement in terms of CPU
>>> usage. So this is not more expensive in terms of CPU cycles.
>>
>> I disagree.
> 
> So do I.
> 
>>
>> However, let's base this disagreement on some evidence. Here is the
>> present 32-bit ARM implementation:
>>
>> 00000048 <vm_get_page_prot>:
>>        48:       e200000f        and     r0, r0, #15
>>        4c:       e3003000        movw    r3, #0
>>                          4c: R_ARM_MOVW_ABS_NC   .LANCHOR1
>>        50:       e3403000        movt    r3, #0
>>                          50: R_ARM_MOVT_ABS      .LANCHOR1
>>        54:       e7930100        ldr     r0, [r3, r0, lsl #2]
>>        58:       e12fff1e        bx      lr
>>
>> That is five instructions long.
> 
> On ppc32 I get:
> 
> 00000094 <vm_get_page_prot>:
>        94:	3d 20 00 00 	lis     r9,0
> 			96: R_PPC_ADDR16_HA	.data..ro_after_init
>        98:	54 84 16 ba 	rlwinm  r4,r4,2,26,29
>        9c:	39 29 00 00 	addi    r9,r9,0
> 			9e: R_PPC_ADDR16_LO	.data..ro_after_init
>        a0:	7d 29 20 2e 	lwzx    r9,r9,r4
>        a4:	91 23 00 00 	stw     r9,0(r3)
>        a8:	4e 80 00 20 	blr
> 
> 
>>
>> Please show that your new implementation is not more expensive on
>> 32-bit ARM. Please do so by building a 32-bit kernel, and providing
>> the disassembly.
> 
> With your series I get:
> 
> 00000000 <vm_get_page_prot>:
>     0:	3d 20 00 00 	lis     r9,0
> 			2: R_PPC_ADDR16_HA	.rodata
>     4:	39 29 00 00 	addi    r9,r9,0
> 			6: R_PPC_ADDR16_LO	.rodata
>     8:	54 84 16 ba 	rlwinm  r4,r4,2,26,29
>     c:	7d 49 20 2e 	lwzx    r10,r9,r4
>    10:	7d 4a 4a 14 	add     r10,r10,r9
>    14:	7d 49 03 a6 	mtctr   r10
>    18:	4e 80 04 20 	bctr
>    1c:	39 20 03 15 	li      r9,789
>    20:	91 23 00 00 	stw     r9,0(r3)
>    24:	4e 80 00 20 	blr
>    28:	39 20 01 15 	li      r9,277
>    2c:	91 23 00 00 	stw     r9,0(r3)
>    30:	4e 80 00 20 	blr
>    34:	39 20 07 15 	li      r9,1813
>    38:	91 23 00 00 	stw     r9,0(r3)
>    3c:	4e 80 00 20 	blr
>    40:	39 20 05 15 	li      r9,1301
>    44:	91 23 00 00 	stw     r9,0(r3)
>    48:	4e 80 00 20 	blr
>    4c:	39 20 01 11 	li      r9,273
>    50:	4b ff ff d0 	b       20 <vm_get_page_prot+0x20>
> 
> 
> That is definitely more expensive, it implements a table of branches.

Okay, will split out the PPC32 implementation that retains existing
table look up method. Also planning to keep that inside same file
(arch/powerpc/mm/mmap.c), unless you have a difference preference.