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SubjectRe: [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.

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