Re: [PATCH] LoongArch: Provide kernel fpu functions

[Huacai Chen <chenhuacai@xxxxxxxxxx>]

Hi, Xuerui,

On Sun, Mar 5, 2023 at 1:53 PM WANG Xuerui <kernel@xxxxxxxxxx> wrote:
>
> On 3/5/23 13:28, Huacai Chen wrote:
> > Provide kernel_fpu_begin()/kernel_fpu_end() to let the kernel use fpu
> > itself. They can be used by AMDGPU graphic driver for DCN.
>
> Grammar nit: "itself" is wrongly placed. "allow the kernel itself to use
> FPU" could be better.
>
> Also the expected usage is way broader than a single driver's single
> component. It's useful for a wide array of operations that will benefit
> from SIMD acceleration support that'll hopefully appear later. For now
> I'd suggest at least adding a single "e.g." after "used by" to signify
> this, if you're not rewording the sentence.
OK, I will update it.

>
> >
> > Reported-by: Xuerui Wang <kernel@xxxxxxxxxx>
> Thanks, but I prefer my name spelled in the native word order ;-)
OK, I will correct it.

> > Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > ---
> >   arch/loongarch/include/asm/fpu.h |  3 +++
> >   arch/loongarch/kernel/Makefile   |  2 +-
> >   arch/loongarch/kernel/kfpu.c     | 41 ++++++++++++++++++++++++++++++++
> >   3 files changed, 45 insertions(+), 1 deletion(-)
> >   create mode 100644 arch/loongarch/kernel/kfpu.c
> >
> > diff --git a/arch/loongarch/include/asm/fpu.h b/arch/loongarch/include/asm/fpu.h
> > index 358b254d9c1d..192f8e35d912 100644
> > --- a/arch/loongarch/include/asm/fpu.h
> > +++ b/arch/loongarch/include/asm/fpu.h
> > @@ -21,6 +21,9 @@
> >
> >   struct sigcontext;
> >
> > +extern void kernel_fpu_begin(void);
> > +extern void kernel_fpu_end(void);
> > +
> >   extern void _init_fpu(unsigned int);
> >   extern void _save_fp(struct loongarch_fpu *);
> >   extern void _restore_fp(struct loongarch_fpu *);
> > diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
> > index 78d4e3384305..9a72d91cd104 100644
> > --- a/arch/loongarch/kernel/Makefile
> > +++ b/arch/loongarch/kernel/Makefile
> > @@ -13,7 +13,7 @@ obj-y               += head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
> >   obj-$(CONFIG_ACPI)          += acpi.o
> >   obj-$(CONFIG_EFI)           += efi.o
> >
> > -obj-$(CONFIG_CPU_HAS_FPU)    += fpu.o
> > +obj-$(CONFIG_CPU_HAS_FPU)    += fpu.o kfpu.o
> >
> >   obj-$(CONFIG_ARCH_STRICT_ALIGN)     += unaligned.o
> >
> > diff --git a/arch/loongarch/kernel/kfpu.c b/arch/loongarch/kernel/kfpu.c
> > new file mode 100644
> > index 000000000000..cd2a18fecdcc
> > --- /dev/null
> > +++ b/arch/loongarch/kernel/kfpu.c
> > @@ -0,0 +1,41 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copyright (C) 2020-2023 Loongson Technology Corporation Limited
> > + */
> > +
> > +#include <linux/cpu.h>
> > +#include <linux/init.h>
> > +#include <asm/fpu.h>
> > +#include <asm/smp.h>
> > +
> > +static DEFINE_PER_CPU(bool, in_kernel_fpu);
> > +
> > +void kernel_fpu_begin(void)
> > +{
> > +     if(this_cpu_read(in_kernel_fpu))
> > +             return;
> Could be a conditional WARN_ON_ONCE like in arch/x86?
> > +
> > +     preempt_disable();
> > +     this_cpu_write(in_kernel_fpu, true);
> > +
> > +     if (!is_fpu_owner())
> > +             enable_fpu();
> > +     else
> > +             _save_fp(&current->thread.fpu);
> > +}
> > +EXPORT_SYMBOL_GPL(kernel_fpu_begin);
>
> Might be good to provide some explanation in the commit message as to
> why the pair of helpers should be GPL-only. Do they touch state buried
> deep enough to make any downstream user a "derivative work"? Or are the
> annotation inspired by arch/x86?
Yes, just inspired by arch/x86, and I don't think these symbols should
be used by non-GPL modules.

Huacai
>
> I think this kinda needs more thought, because similar operations like
> arm's kernel_neon_{begin,end}, powerpc's enable_kernel_{fp,vsx,altivec}
> or s390's __kernel_fpu_{begin,end} are not made GPL-only. Making these
> helpers GPL-only precludes any non-GPL module to make use of SIMD on
> LoongArch, which may or may not be what you want. This can have
> commercial consequences so I can only leave the decision to you.
> (Although IMO the semantics are encapsulated and high-level enough to
> not warrant GPL-only marks, but it may well be the case that you have
> thought of something else but didn't mention here.)
>
> > +
> > +void kernel_fpu_end(void)
> > +{
> > +     if(!this_cpu_read(in_kernel_fpu))
> > +             return;
> > +
> > +     if (!is_fpu_owner())
> > +             disable_fpu();
> > +     else
> > +             _restore_fp(&current->thread.fpu);
> > +
> > +     this_cpu_write(in_kernel_fpu, false);
> > +     preempt_enable();
> > +}
> > +EXPORT_SYMBOL_GPL(kernel_fpu_end);
>
> --
> WANG "xen0n" Xuerui
>
> Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/
>