From: Dave Martin <Dave.Martin@arm.com> To: Vincenzo Frascino <vincenzo.frascino@arm.com> Cc: linux-arch@vger.kernel.org, Shijith Thotton <sthotton@marvell.com>, Peter Collingbourne <pcc@google.com>, Arnd Bergmann <arnd@arndb.de>, Huw Davies <huw@codeweavers.com>, Catalin Marinas <catalin.marinas@arm.com>, Daniel Lezcano <daniel.lezcano@linaro.org>, Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org, Ralf Baechle <ralf@linux-mips.org>, Mark Salyzyn <salyzyn@android.com>, Paul Burton <paul.burton@mips.com>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, linux-kselftest@vger.kernel.org, Andre Przywara <andre.przywara@arm.com>, Russell King <linux@armlinux.org.uk>, Dmitry Safonov <0x7f454c46@gmail.com>, linux-mips@vger.kernel.org, Shuah Khan <shuah@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Date: Thu, 27 Jun 2019 12:27:32 +0100 Message-ID: <20190627112731.GF2790@e103592.cambridge.arm.com> (raw) In-Reply-To: <85808e79-27a0-d3ab-3fb0-445f79ff87a4@arm.com> On Thu, Jun 27, 2019 at 11:57:36AM +0100, Vincenzo Frascino wrote: > Hi Dave, > > Overall, I want to thank you for bringing out the topic. It helped me to > question some decisions and make sure that we have no holes left in > the approach. Fair enough. This is really just a nasty compiler corner-case... the validity of the overall approach isn't affected. > >> > >> vDSO library is a shared object not compiled with LTO as far as I can > >> see, hence if this involved LTO should not applicable in this case. > > > > That turned to be a spurious hypothesis on my part -- LTO isn't the > > smoking gun. (See below.) > > > > Ok. > > >>> The classic example of this (triggered directly and not due to inlining) > >>> would be something like: > >>> > >>> int bar(int, int); > >>> > >>> void foo(int x, int y) > >>> { > >>> register int x_ asm("r0") = x; > >>> register int y_ asm("r1") = bar(x, y); > >>> > >>> asm volatile ( > >>> "svc #0" > >>> :: "r" (x_), "r" (y_) > >>> : "memory" > >>> ); > >>> } > >>> > >>> -> > >>> > >>> 0000000000000000 <foo>: > >>> 0: a9bf7bfd stp x29, x30, [sp, #-16]! > >>> 4: 910003fd mov x29, sp > >>> 8: 94000000 bl 0 <bar> > >>> c: 2a0003e1 mov w1, w0 > >>> 10: d4000001 svc #0x0 > >>> 14: a8c17bfd ldp x29, x30, [sp], #16 > >>> 18: d65f03c0 ret > >>> > >> > >> Contextualized to what my vdso fallback functions do, this should not be a > >> concern because in no case a function result is directly set to a variable > >> declared as register. > >> > >> Since the vdso fallback functions serve a very specific and limited purpose, I > >> do not expect that that code is going to change much in future. > >> > >> The only thing that can happen is something similar to what I wrote in my > >> example, which as I empirically proved does not trigger the problematic behavior. > >> > >>> > >>> The gcc documentation is vague and ambiguous about precisely whan this > >>> can happen and about how to avoid it. > >>> > >> > >> On this I agree, it is not very clear, but this seems more something to raise > >> with the gcc folks in order to have a more "explicit" description that leaves no > >> room to the interpretation. > >> > >> ... > >> > >>> > >>> However, the workaround is cheap, and to avoid the chance of subtle > >>> intermittent code gen bugs it may be worth it: > >>> > >>> void foo(int x, int y) > >>> { > >>> asm volatile ( > >>> "mov x0, %0\n\t" > >>> "mov x1, %1\n\t" > >>> "svc #0" > >>> :: "r" (x), "r" (bar(x, y)) > >>> : "r0", "r1", "memory" > >>> ); > >>> } > >>> > >>> -> > >>> > >>> 0000000000000000 <foo>: > >>> 0: a9be7bfd stp x29, x30, [sp, #-32]! > >>> 4: 910003fd mov x29, sp > >>> 8: f9000bf3 str x19, [sp, #16] > >>> c: 2a0003f3 mov w19, w0 > >>> 10: 94000000 bl 0 <bar> > >>> 14: 2a0003e2 mov w2, w0 > >>> 18: aa1303e0 mov x0, x19 > >>> 1c: aa0203e1 mov x1, x2 > >>> 20: d4000001 svc #0x0 > >>> 24: f9400bf3 ldr x19, [sp, #16] > >>> 28: a8c27bfd ldp x29, x30, [sp], #32 > >>> 2c: d65f03c0 ret > >>> > >>> > >>> What do you think? > >>> > >> > >> The solution seems ok, thanks for providing it, but IMHO I think we > >> should find a workaround for something that is broken, which, unless > >> I am missing something major, this seems not the case. > > > > So, after a bit of further experimentation, I found that I could trigger > > it with implicit function calls on an older compiler. I couldn't show > > it with explicit function calls (as in your example). > > > > With the following code, inlining if an expression that causes an > > implicit call to a libgcc helper can trigger this issue, but I had to > > try an older compiler: > > > > int foo(int x, int y) > > { > > register int res asm("r0"); > > register const int x_ asm("r0") = x; > > register const int y_ asm("r1") = y; > > > > asm volatile ( > > "svc #0" > > : "=r" (res) > > : "r" (x_), "r" (y_) > > : "memory" > > ); > > > > return res; > > } > > > > int bar(int x, int y) > > { > > return foo(x, x / y); > > } > > > > -> (arm-linux-gnueabihf-gcc 9.1 -O2) > > > > 00000000 <foo>: > > 0: df00 svc 0 > > 2: 4770 bx lr > > > > 00000004 <bar>: > > 4: b510 push {r4, lr} > > 6: 4604 mov r4, r0 > > 8: f7ff fffe bl 0 <__aeabi_idiv> > > c: 4601 mov r1, r0 > > e: 4620 mov r0, r4 > > 10: df00 svc 0 > > 12: bd10 pop {r4, pc} > > > > -> (arm-linux-gnueabihf-gcc 5.1 -O2) > > > > 00000000 <foo>: > > 0: df00 svc 0 > > 2: 4770 bx lr > > > > 00000004 <bar>: > > 4: b508 push {r3, lr} > > 6: f7ff fffe bl 0 <__aeabi_idiv> > > a: 4601 mov r1, r0 > > c: df00 svc 0 > > e: bd08 pop {r3, pc} > > > > Thanks for reporting this. I had a go with gcc-5.1 on the vDSO library and seems > Ok, but it was worth trying. > > For obvious reasons I am not reporting the objdump here :) > > > I was struggling to find a way to emit an implicit function call for > > AArch64, except for 128-bit divide, which would complicate things since > > uint128_t doesn't fit in a single register anyway. > > > > Maybe this was considered a bug and fixed sometime after GCC 5, but I > > think the GCC documentation is still quite unclear on the semantics of > > register asm vars that alias call-clobbered registers in the PCS. > > > > If we can get a promise out of the GCC folks that this will not happen > > with any future compiler, then maybe we could just require a new enough > > compiler to be used. > > > > On this I fully agree, the compiler should never change an "expected" behavior. > > If the issue comes from a gray area in the documentation, we have to address it > and have it fixed there. > > The minimum version of the compiler from linux-4.19 is 4.6, hence I had to try > that the vDSO lib does not break with 5.1 [1]. > > [1] > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cafa0010cd51fb711fdcb50fc55f394c5f167a0a OK > > Then of course there is clang. > > > > I could not help myself and I tried clang.8 and clang.7 as well with my example, > just to make sure that we are fine even in that case. Please find below the > results (pretty identical). > > main.clang.7.o: file format ELF64-aarch64-little > > Disassembly of section .text: > 0000000000000000 show_it: > 0: e8 03 1f aa mov x8, xzr > 4: 09 68 68 38 ldrb w9, [x0, x8] > 8: 08 05 00 91 add x8, x8, #1 > c: c9 ff ff 34 cbz w9, #-8 <show_it+0x4> > 10: 02 05 00 51 sub w2, w8, #1 > 14: e1 03 00 aa mov x1, x0 > 18: 08 08 80 d2 mov x8, #64 > 1c: 01 00 00 d4 svc #0 > 20: c0 03 5f d6 ret > > main.clang.8.o: file format ELF64-aarch64-little > > Disassembly of section .text: > 0000000000000000 show_it: > 0: e8 03 1f aa mov x8, xzr > 4: 09 68 68 38 ldrb w9, [x0, x8] > 8: 08 05 00 91 add x8, x8, #1 > c: c9 ff ff 34 cbz w9, #-8 <show_it+0x4> > 10: 02 05 00 51 sub w2, w8, #1 > 14: e1 03 00 aa mov x1, x0 > 18: 08 08 80 d2 mov x8, #64 > 1c: 01 00 00 d4 svc #0 > 20: c0 03 5f d6 ret > > Commands used: > > $ clang -target aarch64-linux-gnueabi main.c -O -c -o main.clang.<x>.o > $ llvm-objdump -d main.clang.<x>.o Actually, I'm not sure this is comparable with the reproducer I quoted in my last reply. The compiler can see the definition of strlen and fully inlines it. I only ever saw the problem when the compiler emits an out-of-line implicit function call. What does clang do with my example on 32-bit? Cheers ---Dave
next prev parent reply index Thread overview: 108+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-06-21 9:52 [PATCH v7 00/25] Unify vDSOs across more architectures Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 01/25] kernel: Standardize vdso_datapage Vincenzo Frascino 2019-06-24 13:56 ` Catalin Marinas 2019-06-21 9:52 ` [PATCH v7 02/25] kernel: Define gettimeofday vdso common code Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 03/25] kernel: Unify update_vsyscall implementation Vincenzo Frascino 2019-06-21 10:49 ` Huw Davies 2019-06-21 9:52 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Vincenzo Frascino 2019-06-24 13:36 ` Will Deacon 2019-06-24 13:59 ` Vincenzo Frascino 2019-06-25 16:18 ` [PATCH 1/3] lib/vdso: Delay mask application in do_hres() Vincenzo Frascino 2019-06-25 16:18 ` [PATCH 2/3] arm64: Fix __arch_get_hw_counter() implementation Vincenzo Frascino 2019-06-25 16:18 ` [PATCH 3/3] arm64: compat: " Vincenzo Frascino 2019-06-25 17:02 ` [PATCH 1/3] lib/vdso: Delay mask application in do_hres() Thomas Gleixner 2019-06-25 18:27 ` Thomas Gleixner 2019-06-25 20:15 ` Andy Lutomirski 2019-06-25 22:24 ` Thomas Gleixner 2019-06-26 6:38 ` Thomas Gleixner 2019-06-26 9:25 ` Vincenzo Frascino 2019-06-26 10:02 ` lib/vdso: Make delta calculation work correctly Thomas Gleixner 2019-06-26 11:08 ` Vincenzo Frascino 2019-06-24 13:58 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Catalin Marinas 2019-06-25 15:33 ` Dave Martin 2019-06-26 13:27 ` Vincenzo Frascino 2019-06-26 16:14 ` Dave Martin 2019-06-26 19:01 ` Vincenzo Frascino 2019-06-27 10:01 ` Dave Martin 2019-06-27 10:57 ` Vincenzo Frascino 2019-06-27 11:27 ` Dave Martin [this message] 2019-06-27 11:59 ` Vincenzo Frascino 2019-06-27 14:38 ` Dave Martin 2019-06-27 15:34 ` Vincenzo Frascino 2019-06-25 17:43 ` [PATCH] arm64: vdso: Fix compilation with clang < 8 Vincenzo Frascino 2019-06-26 11:36 ` [PATCH v2] arm64: vdso: Fix compilation with clang older then 8 Vincenzo Frascino [not found] ` <CGME20190628130921eucas1p239935b0771032c331911eacc1a69dd2e@eucas1p2.samsung.com> 2019-06-28 13:09 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Marek Szyprowski 2019-06-28 14:32 ` Vincenzo Frascino 2019-06-28 16:50 ` Sylwester Nawrocki 2019-06-29 6:58 ` Vincenzo Frascino 2019-07-08 12:57 ` Sylwester Nawrocki 2019-07-08 13:09 ` Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 05/25] arm64: Build vDSO with -ffixed-x18 Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 06/25] arm64: compat: Add missing syscall numbers Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 07/25] arm64: compat: Expose signal related structures Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 08/25] arm64: compat: Generate asm offsets for signals Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 09/25] lib: vdso: Add compat support Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 10/25] arm64: compat: Add vDSO Vincenzo Frascino 2019-06-24 14:00 ` Catalin Marinas 2019-07-10 4:02 ` John Stultz 2019-07-10 6:12 ` Thomas Gleixner 2019-07-10 9:48 ` Vincenzo Frascino 2019-07-10 8:27 ` Will Deacon 2019-07-10 8:58 ` Thomas Gleixner 2019-07-10 9:12 ` Will Deacon 2019-07-10 9:47 ` Vincenzo Frascino 2019-07-10 13:41 ` Vincenzo Frascino 2019-07-10 13:04 ` [PATCH] arm64: vdso: Fix ABI regression in compat vdso Vincenzo Frascino 2019-07-10 13:25 ` Will Deacon 2019-07-10 13:42 ` Vincenzo Frascino 2019-07-10 14:01 ` [PATCH v2] " Vincenzo Frascino 2019-07-10 15:44 ` John Stultz 2019-07-10 15:53 ` Vincenzo Frascino 2019-07-11 9:45 ` Will Deacon 2019-07-11 10:34 ` Thomas Gleixner 2019-07-11 11:32 ` Will Deacon 2019-06-21 9:52 ` [PATCH v7 11/25] arm64: Refactor vDSO code Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 12/25] arm64: compat: vDSO setup for compat layer Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 13/25] arm64: elf: vDSO code page discovery Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 14/25] arm64: compat: Get sigreturn trampolines from vDSO Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 15/25] arm64: Add vDSO compat support Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 16/25] arm: Add support for generic vDSO Vincenzo Frascino 2019-12-04 13:51 ` [PATCH v7 16/25] arm: Add support for generic vDSO (causing crash) Guenter Roeck 2019-12-04 13:58 ` Vincenzo Frascino 2019-12-04 16:16 ` Guenter Roeck 2019-12-04 17:15 ` Vincenzo Frascino 2019-12-04 19:39 ` Guenter Roeck 2019-12-05 9:42 ` Philippe Mathieu-Daudé 2019-12-05 10:00 ` Vincenzo Frascino 2019-12-05 11:02 ` Arnd Bergmann 2019-12-05 14:56 ` Philippe Mathieu-Daudé 2019-06-21 9:52 ` [PATCH v7 17/25] arm: Add clock_getres entry point Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 18/25] arm: Add clock_gettime64 " Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 19/25] mips: Add support for generic vDSO Vincenzo Frascino 2019-07-26 5:15 ` Paul Burton 2019-07-26 16:29 ` [PATCH 0/2] mips: vdso: Fix Makefile Vincenzo Frascino 2019-07-26 16:29 ` [PATCH 1/2] mips: vdso: Fix source path Vincenzo Frascino 2019-07-26 16:29 ` [PATCH 2/2] mips: vdso: Fix flip/flop vdso building bug Vincenzo Frascino 2019-07-28 22:20 ` [PATCH 0/2] mips: vdso: Fix Makefile Paul Burton 2019-06-21 9:52 ` [PATCH v7 20/25] mips: Add clock_getres entry point Vincenzo Frascino 2019-07-26 5:15 ` Paul Burton 2019-06-21 9:52 ` [PATCH v7 21/25] mips: Add clock_gettime64 " Vincenzo Frascino 2019-07-26 5:15 ` Paul Burton 2019-06-21 9:52 ` [PATCH v7 22/25] x86: Add support for generic vDSO Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 23/25] x86: Add clock_getres entry point Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 24/25] x86: Add clock_gettime64 " Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 25/25] kselftest: Extend vDSO selftest Vincenzo Frascino 2019-06-24 0:34 ` [PATCH v7 00/25] Unify vDSOs across more architectures Thomas Gleixner 2019-06-24 1:15 ` Andy Lutomirski 2019-06-24 7:42 ` Thomas Gleixner 2019-06-24 13:21 ` Vincenzo Frascino 2019-06-24 14:18 ` Thomas Gleixner 2019-06-24 14:23 ` Russell King - ARM Linux admin 2019-06-24 14:49 ` Catalin Marinas 2019-06-24 16:20 ` Vincenzo Frascino 2019-10-25 11:42 ` Geert Uytterhoeven 2019-06-24 18:41 ` Paul Burton 2019-06-24 23:16 ` Vincenzo Frascino 2019-06-25 17:11 ` Paul Burton 2019-06-25 17:17 ` Vincenzo Frascino 2019-06-24 12:50 ` Andre Przywara
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