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

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

On 3/2/22 3:35 PM, Geert Uytterhoeven wrote:
> Hi Anshuman,
> 
> On Wed, Mar 2, 2022 at 10:51 AM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>> 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.
> 
> I doubt the switch() variant would give better code on any platform.
> 
> What about using tables everywhere, using designated initializers
> to improve readability?

Designated initializers ? Could you please be more specific. A table look
up on arm platform would be something like this and arm_protection_map[]
needs to be updated with user_pgprot like before. Just wondering how a
designated initializer will help here.

static pgprot_t arm_protection_map[16] __ro_after_init = {
       [VM_NONE]                                       = __PAGE_NONE,
       [VM_READ]                                       = __PAGE_READONLY,
       [VM_WRITE]                                      = __PAGE_COPY,
       [VM_WRITE | VM_READ]                            = __PAGE_COPY,
       [VM_EXEC]                                       = __PAGE_READONLY_EXEC,
       [VM_EXEC | VM_READ]                             = __PAGE_READONLY_EXEC,
       [VM_EXEC | VM_WRITE]                            = __PAGE_COPY_EXEC,
       [VM_EXEC | VM_WRITE | VM_READ]                  = __PAGE_COPY_EXEC,
       [VM_SHARED]                                     = __PAGE_NONE,
       [VM_SHARED | VM_READ]                           = __PAGE_READONLY,
       [VM_SHARED | VM_WRITE]                          = __PAGE_SHARED,
       [VM_SHARED | VM_WRITE | VM_READ]                = __PAGE_SHARED,
       [VM_SHARED | VM_EXEC]                           = __PAGE_READONLY_EXEC,
       [VM_SHARED | VM_EXEC | VM_READ]                 = __PAGE_READONLY_EXEC,
       [VM_SHARED | VM_EXEC | VM_WRITE]                = __PAGE_SHARED_EXEC,
       [VM_SHARED | VM_EXEC | VM_WRITE | VM_READ]      = __PAGE_SHARED_EXEC
};

pgprot_t vm_get_page_prot(unsigned long vm_flags)
{
       return __pgprot(pgprot_val(arm_protection_map[vm_flags &
                      (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]));
}
EXPORT_SYMBOL(vm_get_page_prot);