From 7caa9765465f60b6d88e22264892cee12d971888 Mon Sep 17 00:00:00 2001 From: Puranjay Mohan Date: Fri, 5 Apr 2024 14:24:53 +0000 Subject: ftrace: riscv: move from REGS to ARGS MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This commit replaces riscv's support for FTRACE_WITH_REGS with support for FTRACE_WITH_ARGS. This is required for the ongoing effort to stop relying on stop_machine() for RISCV's implementation of ftrace. The main relevant benefit that this change will bring for the above use-case is that now we don't have separate ftrace_caller and ftrace_regs_caller trampolines. This will allow the callsite to call ftrace_caller by modifying a single instruction. Now the callsite can do something similar to: When not tracing: | When tracing: func: func: auipc t0, ftrace_caller_top auipc t0, ftrace_caller_top nop <==================> jalr t0, ftrace_caller_bottom [...] [...] The above assumes that we are dropping the support of calling a direct trampoline from the callsite. We need to drop this as the callsite can't change the target address to call, it can only enable/disable a call to a preset target (ftrace_caller in the above diagram). We can later optimize this by calling an intermediate dispatcher trampoline before ftrace_caller. Currently, ftrace_regs_caller saves all CPU registers in the format of struct pt_regs and allows the tracer to modify them. We don't need to save all of the CPU registers because at function entry only a subset of pt_regs is live: |----------+----------+---------------------------------------------| | Register | ABI Name | Description | |----------+----------+---------------------------------------------| | x1 | ra | Return address for traced function | | x2 | sp | Stack pointer | | x5 | t0 | Return address for ftrace_caller trampoline | | x8 | s0/fp | Frame pointer | | x10-11 | a0-1 | Function arguments/return values | | x12-17 | a2-7 | Function arguments | |----------+----------+---------------------------------------------| See RISCV calling convention[1] for the above table. Saving just the live registers decreases the amount of stack space required from 288 Bytes to 112 Bytes. Basic testing was done with this on the VisionFive 2 development board. Note: - Moving from REGS to ARGS will mean that RISCV will stop supporting KPROBES_ON_FTRACE as it requires full pt_regs to be saved. - KPROBES_ON_FTRACE will be supplanted by FPROBES see [2]. [1] https://riscv.org/wp-content/uploads/2015/01/riscv-calling.pdf [2] https://lore.kernel.org/all/170887410337.564249.6360118840946697039.stgit@devnote2/ Signed-off-by: Puranjay Mohan Tested-by: Björn Töpel Reviewed-by: Björn Töpel Link: https://lore.kernel.org/r/20240405142453.4187-1-puranjay@kernel.org Signed-off-by: Palmer Dabbelt --- include/linux/ftrace.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'include') diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h index e3a83ebd1b33..800995c425e0 100644 --- a/include/linux/ftrace.h +++ b/include/linux/ftrace.h @@ -819,7 +819,8 @@ static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec) extern int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr); #if defined(CONFIG_DYNAMIC_FTRACE_WITH_REGS) || \ - defined(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS) + defined(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS) || \ + defined(CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS) /** * ftrace_modify_call - convert from one addr to another (no nop) * @rec: the call site record (e.g. mcount/fentry) -- cgit v1.2.3