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 [thread overview]
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 other threads:[~2019-06-27 11:27 UTC|newest]
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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