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.
