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
