From 62447cd7aa17097cefdb0d7a2f1312ff55fcda27 Mon Sep 17 00:00:00 2001 From: Samuel Holland Date: Fri, 25 Oct 2024 11:45:47 -0700 Subject: [PATCH] include: sbi: Optimize reads of mhartid and mscratch csr_read() is marked as volatile and clobbering memory, which is generally the safe thing to do. However, these two CSRs do not have any side effects, and the values returned do not change between calls. The compiler can generate better code if we allow it to reorder calls to these functions and cache the return value. Introduce csr_read_relaxed() for this use case. Signed-off-by: Samuel Holland Reviewed-by: Anup Patel --- include/sbi/riscv_asm.h | 10 +++++++++- include/sbi/sbi_scratch.h | 2 +- 2 files changed, 10 insertions(+), 2 deletions(-) diff --git a/include/sbi/riscv_asm.h b/include/sbi/riscv_asm.h index 2c34635a..4605db20 100644 --- a/include/sbi/riscv_asm.h +++ b/include/sbi/riscv_asm.h @@ -101,6 +101,14 @@ __v; \ }) +/* Variant of csr_read() that allows the compiler to cache the value. */ +#define csr_read_relaxed(csr) \ + ({ \ + register unsigned long __v; \ + __asm__ ("csrr %0, " __ASM_STR(csr) : "=r"(__v)); \ + __v; \ + }) + #define csr_write(csr, val) \ ({ \ unsigned long __v = (unsigned long)(val); \ @@ -163,7 +171,7 @@ void csr_write_num(int csr_num, unsigned long val); } while (0) /* Get current HART id */ -#define current_hartid() ((unsigned int)csr_read(CSR_MHARTID)) +#define current_hartid() ((unsigned int)csr_read_relaxed(CSR_MHARTID)) /* determine CPU extension, return non-zero support */ int misa_extension_imp(char ext); diff --git a/include/sbi/sbi_scratch.h b/include/sbi/sbi_scratch.h index 9ae4f891..9cd7b0d6 100644 --- a/include/sbi/sbi_scratch.h +++ b/include/sbi/sbi_scratch.h @@ -159,7 +159,7 @@ enum sbi_scratch_options { /** Get pointer to sbi_scratch for current HART */ #define sbi_scratch_thishart_ptr() \ - ((struct sbi_scratch *)csr_read(CSR_MSCRATCH)) + ((struct sbi_scratch *)csr_read_relaxed(CSR_MSCRATCH)) /** Get Arg1 of next booting stage for current HART */ #define sbi_scratch_thishart_arg1_ptr() \