Re: [PATCH v3] riscv: Add runtime constant support

[Charlie Jenkins <charlie@rivosinc.com>]

On Wed, Jan 29, 2025 at 04:30:49AM -0500, Emil Renner Berthing wrote:
> Charlie Jenkins wrote:
> > Implement the runtime constant infrastructure for riscv. Use this
> > infrastructure to generate constants to be used by the d_hash()
> > function.
> >
> > This is the riscv variant of commit 94a2bc0f611c ("arm64: add 'runtime
> > constant' support") and commit e3c92e81711d ("runtime constants: add
> > x86 architecture support").
> >
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> > Ard brought this to my attention in this patch [1].
> >
> > [1] https://lore.kernel.org/lkml/CAMj1kXE4DJnwFejNWQu784GvyJO=aGNrzuLjSxiowX_e7nW8QA@mail.gmail.com/
> > ---
> > Changes in v3:
> > - Leverage "pack" instruction for runtime_const_ptr() to reduce hot path
> >   by 3 instructions if Zbkb is supported. Suggested by Pasha Bouzarjomehri (pasha@rivosinc.com)
> > - Link to v2: https://lore.kernel.org/r/20250127-runtime_const_riscv-v2-1-95ae7cf97a39@rivosinc.com
> >
> > Changes in v2:
> > - Treat instructions as __le32 and do proper conversions (Ben)
> > - Link to v1: https://lore.kernel.org/r/20250127-runtime_const_riscv-v1-1-795b023ea20b@rivosinc.com
> > ---
> >  arch/riscv/include/asm/runtime-const.h | 194 +++++++++++++++++++++++++++++++++
> >  arch/riscv/kernel/vmlinux.lds.S        |   3 +
> >  2 files changed, 197 insertions(+)
> >
> > diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..0ecbe6967013900781b0b1048d4622f676b64076
> > --- /dev/null
> > +++ b/arch/riscv/include/asm/runtime-const.h
> > @@ -0,0 +1,194 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _ASM_RISCV_RUNTIME_CONST_H
> > +#define _ASM_RISCV_RUNTIME_CONST_H
> > +
> > +#include <asm/alternative.h>
> > +#include <asm/cacheflush.h>
> > +#include <asm/text-patching.h>
> > +#include <linux/uaccess.h>
> > +
> > +#ifdef CONFIG_32BIT
> > +#define runtime_const_ptr(sym)					\
> > +({								\
> > +	typeof(sym) __ret, __tmp;				\
> > +	asm_inline("1:\t"					\
> > +		".option push"					\
> > +		".option norvc"					\
> > +		"lui	%[__ret],0x89abd\n\t"			\
> > +		"addi	%[__ret],-0x211\n\t"			\
> > +		".option pop"					\
> > +		".pushsection runtime_ptr_" #sym ",\"a\"\n\t"	\
> > +		".long 1b - .\n\t"				\
> > +		".popsection"					\
> > +		: [__ret] "=r" (__ret));			\
> > +	__ret;							\
> > +})
> > +#else
> > +/*
> > + * Loading 64-bit constants into a register from immediates is a non-trivial
> > + * task on riscv64. To get it somewhat performant, load 32 bits into two
> > + * different registers and then combine the results.
> > + *
> > + * If the processor supports the Zbkb extension, we can combine the final
> > + * "slli,slli,srli,add" into the single "pack" instruction. If the processor
> > + * doesn't support Zbkb but does support the Zbb extension, we can
> > + * combine the final "slli,srli,add" into one instruction "add.uw".
> > + */
> > +#define runtime_const_ptr(sym)						\
> > +({									\
> > +	typeof(sym) __ret, __tmp;					\
> > +	asm_inline("1:\t"						\
> > +		".option push\n\t"					\
> > +		".option norvc\n\t"					\
> > +		"lui	%[__ret],0x89abd\n\t"				\
> > +		"lui	%[__tmp],0x1234\n\t"				\
> > +		"addiw	%[__ret],%[__ret],-0x211\n\t"			\
> > +		"addiw	%[__tmp],%[__tmp],0x567\n\t"			\
> > +		ALTERNATIVE_2(						\
> > +			"slli	%[__tmp],%[__tmp],32\n\t"		\
> > +			"slli	%[__ret],%[__ret],32\n\t"		\
> > +			"srli	%[__ret],%[__ret],32\n\t"		\
> > +			"add	%[__ret],%[__ret],%[__tmp]\n\t",	\
> > +			".option push\n\t"				\
> > +			".option arch,+zba\n\t"				\
> > +			"slli	%[__tmp],%[__tmp],32\n\t"		\
> > +			"add.uw %[__ret],%[__ret],%[__tmp]\n\t"		\
> > +			"nop\n\t"					\
> > +			"nop\n\t"					\
> > +			".option pop\n\t",				\
> > +			0, RISCV_ISA_EXT_ZBA, 1,			\
> > +			".option push\n\t"				\
> > +			".option arch,+zbkb\n\t"			\
> > +			"pack	%[__ret],%[__ret],%[__tmp]\n\t"		\
> > +			"nop\n\t"					\
> > +			"nop\n\t"					\
> > +			"nop\n\t"					\
> > +			".option pop\n\t",				\
> > +			0, RISCV_ISA_EXT_ZBKB, 1			\
> > +		)							\
> > +		".option pop\n\t"					\
> > +		".pushsection runtime_ptr_" #sym ",\"a\"\n\t"		\
> > +		".long 1b - .\n\t"					\
> > +		".popsection"						\
> > +		: [__ret] "=r" (__ret), [__tmp] "=r" (__tmp));		\
> > +	__ret;								\
> > +})
> > +#endif
> > +
> > +#ifdef CONFIG_32BIT
> > +#define SRLI "srli "
> > +#else
> > +#define SRLI "srliw "
> > +#endif
> > +
> > +#define runtime_const_shift_right_32(val, sym)			\
> > +({								\
> > +	u32 __ret;						\
> > +	asm_inline("1:\t"					\
> > +		".option push\n\t"				\
> > +		".option norvc\n\t"				\
> > +		SRLI "%[__ret],%[__val],12\n\t"			\
> > +		".option pop\n\t"				\
> > +		".pushsection runtime_shift_" #sym ",\"a\"\n\t"	\
> > +		".long 1b - .\n\t"				\
> > +		".popsection"					\
> > +		: [__ret] "=r" (__ret)				\
> > +		: [__val] "r" (val));				\
> > +	__ret;							\
> > +})
> > +
> > +#define runtime_const_init(type, sym) do {			\
> > +	extern s32 __start_runtime_##type##_##sym[];		\
> > +	extern s32 __stop_runtime_##type##_##sym[];		\
> > +								\
> > +	runtime_const_fixup(__runtime_fixup_##type,		\
> > +			    (unsigned long)(sym),		\
> > +			    __start_runtime_##type##_##sym,	\
> > +			    __stop_runtime_##type##_##sym);	\
> > +} while (0)
> > +
> > +static inline void __runtime_fixup_caches(void *where, unsigned int insns)
> > +{
> > +	/* On riscv there are currently only cache-wide flushes so va is ignored. */
> > +	__always_unused uintptr_t va = (uintptr_t)where;
> > +
> > +	flush_icache_range(va, va + 4*insns);
> > +}
> > +
> > +/*
> > + * The 32-bit immediate is stored in a lui+addi pairing.
> > + * lui holds the upper 20 bits of the immediate in the first 20 bits of the instruction.
> > + * addi holds the lower 12 bits of the immediate in the first 12 bits of the instruction.
> > + */
> > +static inline void __runtime_fixup_32(u32 *lui, u32 *addi, unsigned int val)
> > +{
> > +	unsigned int lower_immediate, upper_immediate;
> > +	u32 lui_insn = le32_to_cpu(*lui);
> > +	u32 addi_insn = le32_to_cpu(*addi);
> 
> Because of the compressed extensions RISC-V instructions are only aligned on
> 16bit boundaries, so is there another reason you know that these two
> instructions are 32bit aligned? Otherwise you're adding unaligned accesses
> here.

Great point, thank you. I will add a ".align 4" to the beginning of
these instructions to force the alignment.

- Charlie

> 
> /Emil
> 
> > +	__le32 addi_res, lui_res;
> > +
> > +	lower_immediate = sign_extend32(val, 11);
> > +	upper_immediate = (val - lower_immediate);
> > +
> > +	if (upper_immediate & 0xfffff000) {
> > +		/* replace upper 20 bits of lui with upper immediate */
> > +		lui_insn &= 0x00000fff;
> > +		lui_insn |= upper_immediate & 0xfffff000;
> > +	} else {
> > +		/* replace lui with nop if immediate is small enough to fit in addi */
> > +		lui_insn = 0x00000013;
> > +	}
> > +
> > +	if (lower_immediate & 0x00000fff) {
> > +		/* replace upper 12 bits of addi with lower 12 bits of val */
> > +		addi_insn &= 0x000fffff;
> > +		addi_insn |= (lower_immediate & 0x00000fff) << 20;
> > +	} else {
> > +		/* replace addi with nop if lower_immediate is empty */
> > +		addi_insn = 0x00000013;
> > +	}
> > +
> > +	addi_res = cpu_to_le32(addi_insn);
> > +	lui_res = cpu_to_le32(lui_insn);
> > +	patch_insn_write(addi, &addi_res, sizeof(addi_res));
> > +	patch_insn_write(lui, &lui_res, sizeof(lui_res));
> > +}
> > +
> > +static inline void __runtime_fixup_ptr(void *where, unsigned long val)
> > +{
> > +	if (IS_ENABLED(CONFIG_32BIT)) {
> > +		__runtime_fixup_32(where, where + 4, val);
> > +		__runtime_fixup_caches(where, 2);
> > +	} else {
> > +		__runtime_fixup_32(where, where + 8, val);
> > +		__runtime_fixup_32(where + 4, where + 12, val >> 32);
> > +		__runtime_fixup_caches(where, 4);
> > +	}
> > +}
> > +
> > +/*
> > + * Replace the least significant 5 bits of the srli/srliw immediate that is
> > + * located at bits 20-24
> > + */
> > +static inline void __runtime_fixup_shift(void *where, unsigned long val)
> > +{
> > +	u32 insn = le32_to_cpu(*(__le32 *)where);
> > +	__le32 res;
> > +
> > +	insn &= 0xfe0fffff;
> > +	insn |= (val & 0b11111) << 20;
> > +
> > +	res = cpu_to_le32(insn);
> > +	patch_text_nosync(where, &res, sizeof(insn));
> > +}
> > +
> > +static inline void runtime_const_fixup(void (*fn)(void *, unsigned long),
> > +				       unsigned long val, s32 *start, s32 *end)
> > +{
> > +	while (start < end) {
> > +		fn(*start + (void *)start, val);
> > +		start++;
> > +	}
> > +}
> > +
> > +#endif /* _ASM_RISCV_RUNTIME_CONST_H */
> > diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> > index 002ca58dd998cb78b662837b5ebac988fb6c77bb..61bd5ba6680a786bf1db7dc37bf1acda0639b5c7 100644
> > --- a/arch/riscv/kernel/vmlinux.lds.S
> > +++ b/arch/riscv/kernel/vmlinux.lds.S
> > @@ -97,6 +97,9 @@ SECTIONS
> >  	{
> >  		EXIT_DATA
> >  	}
> > +
> > +	RUNTIME_CONST_VARIABLES
> > +
> >  	PERCPU_SECTION(L1_CACHE_BYTES)
> >
> >  	.rel.dyn : {
> >
> > ---
> > base-commit: ffd294d346d185b70e28b1a28abe367bbfe53c04
> > change-id: 20250123-runtime_const_riscv-6cd854ee2817
> > --
> > - Charlie
> >
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv

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