Message-ID: <87il6bqfnr.fsf@mail.lhotse>
Date: Thu, 09 Nov 2023 23:23:20 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: Christophe Leroy <christophe.leroy@...roup.eu>, Arnd Bergmann
 <arnd@...db.de>, Arnd
 Bergmann <arnd@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Masahiro
 Yamada <masahiroy@...nel.org>, "linux-kbuild@...r.kernel.org"
 <linux-kbuild@...r.kernel.org>
Cc: Matt Turner <mattst88@...il.com>, Vineet Gupta <vgupta@...nel.org>,
 Russell King <linux@...linux.org.uk>, Catalin Marinas
 <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Steven Rostedt
 <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, Mark
 Rutland <mark.rutland@....com>, guoren <guoren@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org>, Huacai Chen
 <chenhuacai@...nel.org>, Greg Ungerer <gerg@...ux-m68k.org>, Michal Simek
 <monstr@...str.eu>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>, Dinh
 Nguyen <dinguyen@...nel.org>, Nicholas Piggin <npiggin@...il.com>, Geoff
 Levand <geoff@...radead.org>, Palmer Dabbelt <palmer@...belt.com>, Heiko
 Carstens <hca@...ux.ibm.com>, John Paul Adrian Glaubitz
 <glaubitz@...sik.fu-berlin.de>, "David S . Miller" <davem@...emloft.net>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo
 Molnar <mingo@...hat.com>, "x86@...nel.org" <x86@...nel.org>, Helge Deller
 <deller@....de>, Sudip Mukherjee <sudipm.mukherjee@...il.com>, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>, Timur Tabi <timur@...nel.org>,
 Kent Overstreet <kent.overstreet@...ux.dev>, David Woodhouse
 <dwmw2@...radead.org>, "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>, Anil
 S
 Keshavamurthy <anil.s.keshavamurthy@...el.com>, Kees Cook
 <keescook@...omium.org>, Vincenzo Frascino <vincenzo.frascino@....com>,
 Juri
 Lelli <juri.lelli@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Nathan Chancellor <nathan@...nel.org>, Nick
 Desaulniers <ndesaulniers@...gle.com>, Nicolas Schier <nicolas@...sle.eu>,
 Alexander Viro <viro@...iv.linux.org.uk>, Uwe Kleine-König
 <u.kleine-koenig@...gutronix.de>, "linux-alpha@...r.kernel.org"
 <linux-alpha@...r.kernel.org>, "linux-snps-arc@...ts.infradead.org"
 <linux-snps-arc@...ts.infradead.org>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "linux-trace-kernel@...r.kernel.org" <linux-trace-kernel@...r.kernel.org>,
 "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
 "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
 "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
 "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
 "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
 "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
 "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
 "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>, Netdev
 <netdev@...r.kernel.org>, "linux-parisc@...r.kernel.org"
 <linux-parisc@...r.kernel.org>, "linux-usb@...r.kernel.org"
 <linux-usb@...r.kernel.org>, "linux-fbdev@...r.kernel.org"
 <linux-fbdev@...r.kernel.org>, "dri-devel@...ts.freedesktop.org"
 <dri-devel@...ts.freedesktop.org>, "linux-bcachefs@...r.kernel.org"
 <linux-bcachefs@...r.kernel.org>, "linux-mtd@...ts.infradead.org"
 <linux-mtd@...ts.infradead.org>
Subject: Re: [PATCH 15/22] arch: vdso: consolidate gettime prototypes

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> Le 09/11/2023 à 11:18, Michael Ellerman a écrit :
>> "Arnd Bergmann" <arnd@...db.de> writes:
>>> On Wed, Nov 8, 2023, at 19:31, Christophe Leroy wrote:
>>>> Le 08/11/2023 à 13:58, Arnd Bergmann a écrit :
>>>
>>>> powerpc has functions doing more or less the same, they are called
>>>> __c_kernel_clock_gettime() and alike with their prototypes siting in
>>>> arch/powerpc/include/asm/vdso/gettimeofday.h
>>>>
>>>> Should those prototypes be moved to include/vdso/gettime.h too and
>>>> eventually renamed, or are they considered too powerpc specific ?
>>>
>>> I don't actually know, my initial interpretation was that
>>> these function names are part of the user ABI for the vdso,
>>> but I never looked closely enough at how vdso works to
>>> be sure what the actual ABI is.
>> 
>> AFAIK the ABI is just the symbols we export, as defined in the linker
>> script:
>> 
>> /*
>>   * This controls what symbols we export from the DSO.
>>   */
>> VERSION
>> {
>> 	VDSO_VERSION_STRING {
>> 	global:
>> 		__kernel_get_syscall_map;
>> 		__kernel_gettimeofday;
>> 		__kernel_clock_gettime;
>> 		__kernel_clock_getres;
>> 		__kernel_get_tbfreq;
>> 		__kernel_sync_dicache;
>> 		__kernel_sigtramp_rt64;
>> 		__kernel_getcpu;
>> 		__kernel_time;
>> 
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/vdso/vdso64.lds.S?h=v6.6&#n117
>> 
>>> If __c_kernel_clock_gettime() etc are not part of the user-facing
>>> ABI, I think renaming them for consistency with the other
>>> architectures would be best.
>> 
>> The __c symbols are not part of the ABI, so we could rename them.
>> 
>> At the moment though they don't have the same prototype as the generic
>> versions, because we find the VDSO data in asm and pass it to the C
>> functions, eg:
>> 
>> int __c_kernel_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz,
>> 			    const struct vdso_data *vd);
>> 
>> I think we can rework that though, by implementing
>> __arch_get_vdso_data() and getting the vdso_data in C. Then we'd be able
>> to share the prototypes.
>
> I think it would not a been good idea, it would be less performant, for 
> explanation see commit 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e876f0b69dc993e86ca7795e63e98385aa9a7ef3

Ah thanks. I was wondering why you had done it in asm.

It's a pity but you're right that's probably a measurable performance
hit for some of those calls.

cheers

Powered by blists - more mailing lists