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Message-ID: <CAMuHMdUPw_yBMe9XxQHHuf40it3V5mGuOA10KnMpXt124Ay8Tw@mail.gmail.com> Date: Wed, 2 Mar 2022 12:14:26 +0100 From: Geert Uytterhoeven <geert@...ux-m68k.org> To: Anshuman Khandual <anshuman.khandual@....com> Cc: Christophe Leroy <christophe.leroy@...roup.eu>, "Russell King (Oracle)" <linux@...linux.org.uk>, "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>, "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>, "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>, "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>, "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>, "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>, "linux-hexagon@...r.kernel.org" <linux-hexagon@...r.kernel.org>, "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>, Christoph Hellwig <hch@...radead.org>, "linux-snps-arc@...ts.infradead.org" <linux-snps-arc@...ts.infradead.org>, "linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>, Arnd Bergmann <arnd@...db.de>, "linux-um@...ts.infradead.org" <linux-um@...ts.infradead.org>, "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>, "openrisc@...ts.librecores.org" <openrisc@...ts.librecores.org>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org> Subject: Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT Hi Anshuman, On Wed, Mar 2, 2022 at 12:07 PM Anshuman Khandual <anshuman.khandual@....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@....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@...ux-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
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