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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>,
	Andre Przywara <andre.przywara@arm.com>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Will Deacon <will.deacon@arm.com>,
	linux-mips@vger.kernel.org, Ralf Baechle <ralf@linux-mips.org>,
	linux-kernel@vger.kernel.org, Paul Burton <paul.burton@mips.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	linux-kselftest@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Russell King <linux@armlinux.org.uk>,
	Dmitry Safonov <0x7f454c46@gmail.com>,
	Mark Salyzyn <salyzyn@android.com>, 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 15:38:26 +0100
Message-ID: <20190627143826.GG2790@e103592.cambridge.arm.com> (raw)
In-Reply-To: <a07b66cb-186f-a743-4f1d-41227f23db74@arm.com>

On Thu, Jun 27, 2019 at 12:59:07PM +0100, Vincenzo Frascino wrote:
> On 6/27/19 12:27 PM, Dave Martin wrote:
> > On Thu, Jun 27, 2019 at 11:57:36AM +0100, Vincenzo Frascino wrote:

[...]

> >> 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.
> >
> 
> As explained in my previous email, this is the only case that can realistically
> happen. vDSO has no dependency on any other library (i.e. libgcc you were
> mentioning) and we are referring to the fallbacks which fall in this category.

Outlining could also introduce a local function call where none exists
explicitly in the program IIUC.

My point is that the interaction between asm reg vars and machine-level
procedure calls is at best ill-defined, and it is largely up to the
compiler when to introduce such a call, even without LTO etc.

So we should not be surprised to see variations in behaviour depending
on compiler, compiler version and compiler flags.

> > 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?
> 
> When clang is selected compat vDSOs are currently disabled on arm64, will be
> introduced with a future patch series.
> 
> Anyway since I am curious as well, this is what happens with your example with
> clang.8 target=arm-linux-gnueabihf:
> 
> dave-code.clang.8.o:	file format ELF32-arm-little
> 
> Disassembly of section .text:
> 0000000000000000 foo:
>        0:	00 00 00 ef 	svc	#0
>        4:	1e ff 2f e1 	bx	lr
> 
> 0000000000000008 bar:
>        8:	10 4c 2d e9 	push	{r4, r10, r11, lr}
>        c:	08 b0 8d e2 	add	r11, sp, #8
>       10:	00 40 a0 e1 	mov	r4, r0
>       14:	fe ff ff eb 	bl	#-8 <bar+0xc>
>       18:	00 10 a0 e1 	mov	r1, r0
>       1c:	04 00 a0 e1 	mov	r0, r4
>       20:	00 00 00 ef 	svc	#0
>       24:	10 8c bd e8 	pop	{r4, r10, r11, pc}

> Compiled with -O2, -O3, -Os never inlines.

Looks sane, and is the behaviour we want.

> Same thing happens for aarch64-linux-gnueabi:
> 
> dave-code.clang.8.o:	file format ELF64-aarch64-little
> 
> Disassembly of section .text:
> 0000000000000000 foo:
>        0:	e0 03 00 2a 	mov	w0, w0
>        4:	e1 03 01 2a 	mov	w1, w1
>        8:	01 00 00 d4 	svc	#0
>        c:	c0 03 5f d6 	ret
> 
> 0000000000000010 bar:
>       10:	01 0c c1 1a 	sdiv	w1, w0, w1
>       14:	e0 03 00 2a 	mov	w0, w0
>       18:	01 00 00 d4 	svc	#0
>       1c:	c0 03 5f d6 	ret

Curious, clang seems to be inserting some seemingly redundant moves
of its own here, though this shouldn't break anything.

I suspect that clang might require an X-reg holding an int to have its
top 32 bits zeroed for passing to an asm, whereas GCC does not.  I think
this comes under "we should not be surprised to see variations".

GCC 9 does this instead:

0000000000000000 <foo>:
   0:   d4000001        svc     #0x0
   4:   d65f03c0        ret

0000000000000008 <bar>:
   8:   1ac10c01        sdiv    w1, w0, w1
   c:   d4000001        svc     #0x0
  10:   d65f03c0        ret


> Based on this I think we can conclude our investigation.

So we use non-reg vars and use the asm clobber list and explicit moves
to get things into / out of the right registers?

Cheers
---Dave

  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
2019-06-27 11:59                 ` Vincenzo Frascino
2019-06-27 14:38                   ` Dave Martin [this message]
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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