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

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Anshuman,

On Wed, Mar 2, 2022 at 12:07 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
> On 3/2/22 3:35 PM, Geert Uytterhoeven wrote:
> > 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.

It's more readable than the original:

    pgprot_t protection_map[16] __ro_after_init = {
            __P000, __P001, __P010, __P011, __P100, __P101, __P110, __P111,
            __S000, __S001, __S010, __S011, __S100, __S101, __S110, __S111
    };

>
> 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
> };

Yeah, like that.

Seems like you already made such a conversion in
https://lore.kernel.org/all/1645425519-9034-3-git-send-email-anshuman.khandual@arm.com/

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds