[PATCH v4 3/3] riscv: store percpu offset into thread_info

Tue May 5 00:20:02 PDT 2026

> diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
> index e9e8ba83e632..137a49488325 100644
> --- a/arch/riscv/include/asm/asm.h
> +++ b/arch/riscv/include/asm/asm.h
> @@ -91,11 +91,7 @@
>
>  #ifdef CONFIG_SMP
>  .macro asm_per_cpu dst sym tmp
> -	lw    \tmp, TASK_TI_CPU_NUM(tp)
> -	slli  \tmp, \tmp, RISCV_LGPTR
> -	la    \dst, __per_cpu_offset
> -	add   \dst, \dst, \tmp
> -	REG_L \tmp, 0(\dst)
> +	REG_L \tmp, TASK_TI_PCPU_OFFSET(tp)
>  	la    \dst, \sym
>  	add   \dst, \dst, \tmp
>  .endm

[ ... ]

> diff --git a/arch/riscv/include/asm/percpu.h b/arch/riscv/include/asm/percpu.h
> index 3b26fe45e70f..84612d672105 100644
> --- a/arch/riscv/include/asm/percpu.h
> +++ b/arch/riscv/include/asm/percpu.h
> @@ -7,7 +7,9 @@
>
>  #include <asm/alternative-macros.h>
>  #include <asm/cpufeature-macros.h>
> +#include <asm/current.h>
>  #include <asm/hwcap.h>
> +#include <asm/thread_info.h>

[ ... ]

> @@ -275,6 +277,8 @@ _pcp_protect_return(__percpu_add_return_amo_case_64, pcp, val)
>  })
>  #endif
>
> +#define __my_cpu_offset (((struct thread_info *)current)->pcpu_offset)
> +
>  #include <asm-generic/percpu.h>

[ ... ]

> diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
> index 0e71eb82f920..733b6cd306e4 100644
> --- a/arch/riscv/include/asm/switch_to.h
> +++ b/arch/riscv/include/asm/switch_to.h
> @@ -88,6 +88,13 @@ static inline void __switch_to_envcfg(struct task_struct *next)
>  			:: "r" (next->thread.envcfg) : "memory");
>  }
>
> +static inline void __switch_to_pcpu_offset(struct task_struct *next)
> +{
> +#ifdef CONFIG_SMP
> +	next->thread_info.pcpu_offset = __my_cpu_offset;
> +#endif
> +}
> +

[ ... ]

> @@ -122,6 +129,7 @@ do {							\
>  	if (switch_to_should_flush_icache(__next))	\
>  		local_flush_icache_all();		\
>  	__switch_to_envcfg(__next);			\
> +	__switch_to_pcpu_offset(__next);		\
>  	((last) = __switch_to(__prev, __next));		\
>  } while (0)

[ ... ]

> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
> index 36918c9200c9..17996dc094fd 100644
> --- a/arch/riscv/include/asm/thread_info.h
> +++ b/arch/riscv/include/asm/thread_info.h
> @@ -53,6 +53,7 @@
>  struct thread_info {
>  	unsigned long		flags;		/* low level flags */
>  	int                     preempt_count;  /* 0=>preemptible, <0=>BUG */
> +	int			cpu;
>  	/*
>  	 * These stack pointers are overwritten on every system call or
>  	 * exception.  SP is also saved to the stack it can be recovered when
> @@ -60,8 +61,8 @@ struct thread_info {
>  	 */
>  	long			kernel_sp;	/* Kernel stack pointer */
>  	long			user_sp;	/* User stack pointer */
> -	int			cpu;
>  	unsigned long		syscall_work;	/* SYSCALL_WORK_ flags */
> +	unsigned long		pcpu_offset;

[ ... ]

> diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> index af827448a609..fbf53b66b0e0 100644
> --- a/arch/riscv/kernel/asm-offsets.c
> +++ b/arch/riscv/kernel/asm-offsets.c
> @@ -38,6 +38,7 @@ void asm_offsets(void)
>  	OFFSET(TASK_THREAD_SUM, task_struct, thread.sum);
>
>  	OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
> +	OFFSET(TASK_TI_PCPU_OFFSET, task_struct, thread_info.pcpu_offset);
>  	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);

[ ... ]

> diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
> index d85916a3660c..9e95c068b966 100644
> --- a/arch/riscv/kernel/smpboot.c
> +++ b/arch/riscv/kernel/smpboot.c
> @@ -209,6 +209,11 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
>  }
>  #endif
>
> +void __init smp_prepare_boot_cpu(void)
> +{
> +	__my_cpu_offset = per_cpu_offset(smp_processor_id());
> +}
> +

[ ... ]

> @@ -234,6 +239,8 @@ asmlinkage __visible void smp_callin(void)
>  	mmgrab(mm);
>  	current->active_mm = mm;
>
> +	__my_cpu_offset = per_cpu_offset(smp_processor_id());
> +

[ ... ]

> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index 2f1109dbf105..177c19216013 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -1395,15 +1395,8 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>  			if (rd != rs)
>  				emit_mv(rd, rs, ctx);
>  #ifdef CONFIG_SMP
> -			/* Load current CPU number in T1 */
> -			emit_lw(RV_REG_T1, offsetof(struct thread_info, cpu),
> +			emit_ld(RV_REG_T1, offsetof(struct thread_info, pcpu_offset),
>  				RV_REG_TP, ctx);
> -			/* Load address of __per_cpu_offset array in T2 */
> -			emit_addr(RV_REG_T2, (u64)&__per_cpu_offset, extra_pass, ctx);
> -			/* Get address of __per_cpu_offset[cpu] in T1 */
> -			emit_sh3add(RV_REG_T1, RV_REG_T1, RV_REG_T2, ctx);
> -			/* Load __per_cpu_offset[cpu] in T1 */
> -			emit_ld(RV_REG_T1, 0, RV_REG_T1, ctx);
>  			/* Add the offset to Rd */
>  			emit_add(rd, rd, RV_REG_T1, ctx);
>  #endif

Note: A lore search found an unaddressed review comment from Arnd Bergmann on
v4 of this patch series, dated Tue, 05 May 2026 08:33:04 +0200. The comment
references arch/riscv/include/asm/io.h regarding null-pointer arithmetic in
PIO helpers. However, that file is not modified in this commit. This may be
feedback on a different patch in the series.

Reference: https://lore.kernel.org/all/3720c3a4-cc0d-4ba8-97ae-35def0189e2e@app.fastmail.com/

---
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CI run summary: https://github.com/kernel-patches/bpf/actions/runs/25361792504