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

Anshuman Khandual
2022-03-02T09:51:37Z

On 3/2/22 12:35 PM, Christophe Leroy wrote:
>
>
> Le 02/03/2022 à 04:22, Anshuman Khandual a écrit :
>>
>>
>> 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.
>
> My point was not to get something specific for PPC32, but to amplify on
> Russell's objection.
>
> As this is bad for ARM and bad for PPC32, do we have any evidence that
> your change is good for any other architecture ?
>
> I checked PPC64 and there is exactly the same drawback. With the current
> implementation it is a small function performing table read then a few
> adjustment. After your change it is a bigger function implementing a
> table of branches.

I am wondering if this would not be the case for any other switch case
statement on the platform ? Is there something specific/different just
on vm_get_page_prot() implementation ? Are you suggesting that switch
case statements should just be avoided instead ?

>
> So, as requested by Russell, could you look at the disassembly for other
> architectures and show us that ARM and POWERPC are the only ones for
> which your change is not optimal ?

But the primary purpose of this series is not to guarantee optimized
code on platform by platform basis, while migrating from a table based
look up method into a switch case statement.

But instead, the purposes is to remove current levels of unnecessary
abstraction while converting a vm_flags access combination into page
protection. The switch case statement for platform implementation of
vm_get_page_prot() just seemed logical enough. Christoph's original
suggestion patch for x86 had the same implementation as well.

But if the table look up is still better/preferred method on certain
platforms like arm or ppc32, will be happy to preserve that.

- Anshuman