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Date: Tue, 02 Apr 2024 15:02:41 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Puranjay Mohan <puranjay12@...il.com>, Alexandre Ghiti
<alexghiti@...osinc.com>, Andy Chiu <andy.chiu@...ive.com>, Paul Walmsley
<paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
<aou@...s.berkeley.edu>, Steven Rostedt <rostedt@...dmis.org>, Masami
Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, Sami
Tolvanen <samitolvanen@...gle.com>, Guo Ren <guoren@...nel.org>, Ley Foon
Tan <leyfoon.tan@...rfivetech.com>, Deepak Gupta <debug@...osinc.com>, Sia
Jee Heng <jeeheng.sia@...rfivetech.com>, Song Shuai
<suagrfillet@...il.com>, Björn Töpel
<bjorn@...osinc.com>, Clément Léger
<cleger@...osinc.com>, Al Viro <viro@...iv.linux.org.uk>,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org
Cc: puranjay12@...il.com
Subject: Re: [RFC PATCH] ftrace: riscv: move from REGS to ARGS
Puranjay Mohan <puranjay12@...il.com> writes:
> 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 <=========<Enable/Disable>=========> 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).
>
> 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.
..and FPROBES, but Masami is (still?) working on a series to address
that [1].
My perspective; This is following the work Mark et al has done for
arm64, and it does make sense for RISC-V as well. I'm in favor having
this change as part of the bigger call_ops/text patch change for RISC-V.
[...]
> diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> index 1276d7d9ca8b..1e95bf4ded6c 100644
> --- a/arch/riscv/include/asm/ftrace.h
> +++ b/arch/riscv/include/asm/ftrace.h
> @@ -124,20 +124,87 @@ struct dyn_ftrace;
> int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
> #define ftrace_init_nop ftrace_init_nop
>
> -#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
> +#define arch_ftrace_get_regs(regs) NULL
> struct ftrace_ops;
> -struct ftrace_regs;
> +struct ftrace_regs {
> + unsigned long epc;
> + unsigned long ra;
> + unsigned long sp;
> + unsigned long s0;
> + unsigned long t1;
> + union {
> + unsigned long args[8];
> + struct {
> + unsigned long a0;
> + unsigned long a1;
> + unsigned long a2;
> + unsigned long a3;
> + unsigned long a4;
> + unsigned long a5;
> + unsigned long a6;
> + unsigned long a7;
> + };
> + };
> +};
> +
> +static __always_inline unsigned long
> +ftrace_regs_get_instruction_pointer(const struct ftrace_regs *fregs)
> +{
> + return fregs->epc;
> +}
> +
> +static __always_inline void
> +ftrace_regs_set_instruction_pointer(struct ftrace_regs *fregs,
> + unsigned long pc)
> +{
> + fregs->epc = pc;
> +}
> +
> +static __always_inline unsigned long
> +ftrace_regs_get_stack_pointer(const struct ftrace_regs *fregs)
> +{
> + return fregs->sp;
> +}
> +
> +static __always_inline unsigned long
> +ftrace_regs_get_argument(struct ftrace_regs *fregs, unsigned int n)
> +{
> + if (n < 8)
> + return fregs->args[n];
> + return 0;
> +}
> +
> +static __always_inline unsigned long
> +ftrace_regs_get_return_value(const struct ftrace_regs *fregs)
> +{
> + return fregs->a0;
> +}
> +
> +static __always_inline void
> +ftrace_regs_set_return_value(struct ftrace_regs *fregs,
> + unsigned long ret)
> +{
> + fregs->a0 = ret;
> +}
> +
> +static __always_inline void
> +ftrace_override_function_with_return(struct ftrace_regs *fregs)
> +{
> + fregs->epc = fregs->ra;
> +}
Style/nit: All above; Try to use the full 100 chars, and keep the
function name return value on the same line for grepability.
Björn
[1] https://lore.kernel.org/all/170887410337.564249.6360118840946697039.stgit@devnote2/
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