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

[Christophe Leroy]

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.


Christophe