linux-kernel - Re: [PATCH 08/12] tools/nolibc: gettimeofday(): avoid libgcc 64-bit divisions

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Message-ID: <69c383ef-0d05-4f9a-bdd1-85dbf3d0100b@t-8ch.de>
Date: Sun, 2 Nov 2025 10:27:18 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Willy Tarreau <w@....eu>
Cc: Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 08/12] tools/nolibc: gettimeofday(): avoid libgcc 64-bit
 divisions

On 2025-11-02 09:31:56+0100, Willy Tarreau wrote:
> On Wed, Oct 29, 2025 at 05:02:58PM +0100, Thomas Weißschuh wrote:
> > timespec::tv_nsec is going to be 64-bit wide even on 32-bit
> > architectures. As not all architectures support 64-bit division
> > instructions, calls to libgcc (__divdi3()) may be emitted by the
> > compiler which are not provided by nolibc.
> > 
> > As tv_nsec is guaranteed to always fit into an uint32_t, perform a
> > 32-bit division instead.
> >
> > Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> > ---
> >  tools/include/nolibc/sys/time.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tools/include/nolibc/sys/time.h b/tools/include/nolibc/sys/time.h
> > index 33782a19aae9..6dd3705c6c9d 100644
> > --- a/tools/include/nolibc/sys/time.h
> > +++ b/tools/include/nolibc/sys/time.h
> > @@ -33,7 +33,7 @@ int sys_gettimeofday(struct timeval *tv, struct timezone *tz)
> >  	ret = sys_clock_gettime(CLOCK_REALTIME, &tp);
> >  	if (!ret && tv) {
> >  		tv->tv_sec = tp.tv_sec;
> > -		tv->tv_usec = tp.tv_nsec / 1000;
> > +		tv->tv_usec = (uint32_t)tp.tv_nsec / 1000;
> >  	}
> 
> Good catch! I'm wondering if this one shouldn't be marked as a build
> fix for 5e7392dc82ed ("tools/nolibc: fall back to sys_clock_gettime()
> in gettimeofday()") so that it can be backported.

Right now timespec::tv_nsec is of type 'long', so it should only be
64-bits on architectures which have native 64-bit division instructions. 
But marking it as fix shouldn't hurt either.


Thomas