Re: [RFC PATCH] ftrace: riscv: move from REGS to ARGS

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 02 Apr 2024 15:02:41 +0200
Björn Töpel <bjorn@kernel.org> wrote:

> Puranjay Mohan <puranjay12@gmail.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].

Yes, even with this patch, FPROBE can be worked with my series.
So I'm OK for this change.

Thank you,

> 
> 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/


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>