linux-kernel - Re: [PATCH v3] LoongArch/percpu: Simplify _percpu_read() and _percpu

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Message-ID: <CAFULd4b9pV2Rpb+X8gTsDdePtcAppmh7jDVcXBQsk39sicRbhw@mail.gmail.com>
Date: Thu, 5 Sep 2024 14:46:22 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Xi Ruoyao <xry111@...111.site>
Cc: Huacai Chen <chenhuacai@...nel.org>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, WANG Xuerui <kernel@...0n.name>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3] LoongArch/percpu: Simplify _percpu_read() and _percpu_write()

On Thu, Sep 5, 2024 at 2:16 PM Uros Bizjak <ubizjak@...il.com> wrote:
>
> On Thu, Sep 5, 2024 at 2:09 PM Xi Ruoyao <xry111@...111.site> wrote:
> >
> > On Thu, 2024-09-05 at 14:02 +0200, Uros Bizjak wrote:
> > > > If the input value is less than 0xff, then "& 0xff" is meaningless, if
> > > > the input value is more than 0xff, this conversion still cannot give a
> > > > correct result for the caller. So I think for all sizes it is enough
> > > > to just use "((unsigned long) val)".
> > >
> > > This part is used to force unsigned extension, otherwise the compiler
> > > will use sign-extension of the possibly signed variable.
> >
> > It's not relevant.  For example when size is 2 __pcpu_op_##size("stx")
> > is expanded to stx.h, and stx.h only stores the lower 16 bits of a
> > register into MEM[r21 + ptr], the high bits are ignored anyway.
> >
> > Thus we can just have
> >
> > +#define _percpu_write(size, _pcp, _val)                                        \
> > +do {                                                                   \
> > +       if (0) {                                                        \
> > +               typeof(_pcp) pto_tmp__;                                 \
> > +               pto_tmp__ = (_val);                                     \
> > +               (void)pto_tmp__;                                        \
> > +       }                                                               \
> > +       __asm__ __volatile__(                                           \
> > +               __pcpu_op_##size("stx") "%[val], $r21, %[ptr]   \n"     \
> > +               :                                                       \
> > +               : [val] "r"(_val), [ptr] "r"(&(_pcp))           \
> > +               : "memory");                                            \
> > +} while (0)
>
> Nice, the less code, the better. If it works for loongson target, then
> we don't need this paranoia.
>
> I just played safe and took the approach that x86 took.

Please note that the original code extended the value to a long type.
If the simplified macro works, then the usage of macros will result in
a better assembly code, where zero-extends will be omitted.

Uros.