linux-kernel - Re: [PATCH] sysfs: make sysfs_emit() return ssize

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Date: Mon, 11 Mar 2024 07:55:22 +0300
From: Alexey Dobriyan <adobriyan@...il.com>
To: David Laight <David.Laight@...lab.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sysfs: make sysfs_emit() return ssize_t

On Sun, Mar 10, 2024 at 07:21:32PM +0000, David Laight wrote:
> From: Alexey Dobriyan
> > Sent: 08 March 2024 06:26
> > 
> > On Thu, Mar 07, 2024 at 10:04:41PM +0000, Greg Kroah-Hartman wrote:
> > > On Mon, Feb 05, 2024 at 01:11:36PM +0300, Alexey Dobriyan wrote:
> > > > sysfs_emit() is most often found in functions returning ssize_t
> > > > not int:
> > > >
> > > > 	static ssize_t oops_count_show(...)
> > > > 	{
> > > > 		return sysfs_emit(page, ...);
> > > > 	}
> > > >
> > > > This pattern results in sign-extension instruction between
> > > > sysfs_emit() return value (int) and caller return value (which is
> > > > ssize_t).
> > >
> > > Is that a problem?
> > 
> > Small problem, but, yes.
> > 
> > If sysfs_emit() returns "int", then every user compiles to
> > 
> > 	call	sysfs_emit
> > 	movsx	rax, eax
> > 	ret
> > 
> > Given than sysfs_emit() is the official way to print in sysfs,
> > there are lots of users and there will be more users in the future
> > as it grows.
> > 
> > This trailing "movsx" instruction is duplicated every time.
> 
> Actually you are missing a tail call...
> But no one will notice the extra execution time, it will
> be masked by the cost of the formatting.
> So it is a small amount of I-cache.
> 
> In reality the best fix is not to use ssize_t for something
> where the domain of the value is much smaller.
> In this case it is probably unsigned as well.

It is much harder to switch read hooks from returning ssize_t to int.
Kernel snprintf() too.

sysfs_emit() change will be mostly seamless, but function pointers
generate warnings.