Re: [PATCH v2] LoongArch: Add jump-label implementation

[Youling Tang]

Hi, Peter

On 05/10/2023 05:27 PM, Peter Zijlstra wrote:
> On Wed, May 10, 2023 at 05:16:46PM +0800, Youling Tang wrote:
> > Add jump-label implementation based on the ARM64 version.
> >
> > Signed-off-by: Youling Tang <tangyouling@xxxxxxxxxxx>
>
> > diff --git a/arch/loongarch/include/asm/jump_label.h b/arch/loongarch/include/asm/jump_label.h
> > new file mode 100644
> > index 000000000000..2f9fdec256c5
> > --- /dev/null
> > +++ b/arch/loongarch/include/asm/jump_label.h
> > @@ -0,0 +1,51 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +/*
> > + * Copyright (C) 2023 Loongson Technology Corporation Limited
> > + *
> > + * Based on arch/arm64/include/asm/jump_label.h
> > + */
> > +#ifndef __ASM_JUMP_LABEL_H
> > +#define __ASM_JUMP_LABEL_H
> > +
> > +#ifndef __ASSEMBLY__
> > +
> > +#include <linux/types.h>
> > +
> > +#define JUMP_LABEL_NOP_SIZE	4
> > +
> > +static __always_inline bool arch_static_branch(struct static_key * const key,
> > +					       const bool branch)
> > +{
> > +	asm_volatile_goto(
> > +		"1:	nop					\n\t"
> > +		 "	.pushsection	__jump_table, \"aw\"	\n\t"
> > +		 "	.align		3			\n\t"
> > +		 "	.long		1b - ., %l[l_yes] - .	\n\t"
> > +		 "	.quad		%0 - .			\n\t"
> > +		 "	.popsection				\n\t"
> > +		 :  :  "i"(&((char *)key)[branch]) :  : l_yes);
> > +
> > +	return false;
> > +l_yes:
> > +	return true;
> > +}
> > +
> > +static __always_inline bool arch_static_branch_jump(struct static_key * const key,
> > +						    const bool branch)
> > +{
> > +	asm_volatile_goto(
> > +		"1:	b		%l[l_yes]		\n\t"
> > +		 "	.pushsection	__jump_table, \"aw\"	\n\t"
> > +		 "	.align		3			\n\t"
> > +		 "	.long		1b - ., %l[l_yes] - .	\n\t"
> > +		 "	.quad		%0 - .			\n\t"
> > +		 "	.popsection				\n\t"
> > +		 :  :  "i"(&((char *)key)[branch]) :  : l_yes);
> > +
> > +	return false;
> > +l_yes:
> > +	return true;
> > +}
>
> Seems simple enough; one change I did a while ago for the x86 version is
> to put the __jump_table entry generation in a macro so it could be
> shared between the (3 for x86) variants.

Looks better, I will define JUMP_TABLE_ENTRY macro so that
arch_static_branch and arch_static_branch_jump can share.

Thanks,
Youling.

> Not saying you have to do that, just saying it's an option.
>
> > +#endif  /* __ASSEMBLY__ */
> > +#endif	/* __ASM_JUMP_LABEL_H */
>
> > diff --git a/arch/loongarch/kernel/jump_label.c b/arch/loongarch/kernel/jump_label.c
> > new file mode 100644
> > index 000000000000..b06245955f7a
> > --- /dev/null
> > +++ b/arch/loongarch/kernel/jump_label.c
> > @@ -0,0 +1,23 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copyright (C) 2023 Loongson Technology Corporation Limited
> > + *
> > + * Based on arch/arm64/kernel/jump_label.c
> > + */
> > +#include <linux/jump_label.h>
> > +#include <linux/kernel.h>
> > +#include <asm/inst.h>
> > +
> > +void arch_jump_label_transform(struct jump_entry *entry,
> > +			       enum jump_label_type type)
> > +{
> > +	void *addr = (void *)jump_entry_code(entry);
> > +	u32 insn;
> > +
> > +	if (type == JUMP_LABEL_JMP)
> > +		insn = larch_insn_gen_b(jump_entry_code(entry), jump_entry_target(entry));
> > +	else
> > +		insn = larch_insn_gen_nop();
> > +
> > +	larch_insn_patch_text(addr, insn);
> > +}
>
> This all implies Loongarch is fine with the nop<->b transition (much
> like arm64 is), but I found no actual mention of what transitions are
> valid for the architecture in your inst.c file -- perhaps you could put
> a small comment there to elucidate the occasional reader that doesn't
> have your arch manual memorized?
>
>
> Anyway, as with most RISC implementations it's short and sweet.
>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>