linux-kernel - Re: [PATCH v2 4/4] ftrace: arm64: move from REGS to ARGS

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Message-ID: <Y3ePYOxwPbSzxhoZ@FVFF77S0Q05N.cambridge.arm.com>
Date:   Fri, 18 Nov 2022 13:57:52 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org, mhiramat@...nel.org,
        revest@...omium.org, rostedt@...dmis.org
Subject: Re: [PATCH v2 4/4] ftrace: arm64: move from REGS to ARGS

On Fri, Nov 18, 2022 at 12:31:50PM +0000, Will Deacon wrote:
> On Thu, Nov 17, 2022 at 10:52:15AM +0000, Mark Rutland wrote:
> > On Tue, Nov 15, 2022 at 11:27:03AM +0000, Will Deacon wrote:
> > > On Thu, Nov 03, 2022 at 05:05:20PM +0000, Mark Rutland wrote:
> > > > This commit replaces arm64's support for FTRACE_WITH_REGS with support
> > > > for FTRACE_WITH_ARGS. This removes some overhead and complexity, and
> > > > removes some latent issues with inconsistent presentation of struct
> > > > pt_regs (which can only be reliably saved/restored at exception
> > > > boundaries).
> > > 
> > > [...]
> > > 
> > > > @@ -78,10 +77,71 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
> > > >  	return addr;
> > > >  }
> > > >  
> > > > -#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > > > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
> > > >  struct dyn_ftrace;
> > > >  struct ftrace_ops;
> > > > -struct ftrace_regs;
> > > > +
> > > > +#define arch_ftrace_get_regs(regs) NULL
> > > > +
> > > > +struct ftrace_regs {
> > > > +	/* x0 - x8 */
> > > > +	unsigned long regs[9];
> > > > +	unsigned long __unused;
> > > > +
> > > > +	unsigned long fp;
> > > > +	unsigned long lr;
> > > > +
> > > > +	unsigned long sp;
> > > > +	unsigned long pc;
> > > > +};
> > > > +
> > > > +static __always_inline unsigned long
> > > > +ftrace_regs_get_instruction_pointer(const struct ftrace_regs *fregs)
> > > > +{
> > > > +	return fregs->pc;
> > > > +}
> > > > +
> > > > +static __always_inline void
> > > > +ftrace_regs_set_instruction_pointer(struct ftrace_regs *fregs,
> > > > +				    unsigned long pc)
> > > > +{
> > > > +	fregs->pc = 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->regs[n];
> > > 
> > > Where does this '8' come from?
> > 
> > Because in AAPCS64 the arguments are in x0 to x7, as mentioned in the commit
> > message:
> > 
> > | Per AAPCS64, all function call argument and return values are held in
> > | the following GPRs:
> > | 
> > | * X0 - X7 : parameter / result registers
> > | * X8      : indirect result location register
> > | * SP      : stack pointer (AKA SP)
> > 
> > The 'indirect result location register' would be used when returning large
> > structures, and isn't a function argument as such.
> 
> Ah gotcha, I was mainly wondering about the role of x8 in 'struct
> ftrace_regs', but now I see that the FETCH_OP_REG might want to get at that.

Ah, I see. Should I just add the bits above from the commit message into a
comment block above the definition of struct ftrace_regs ?

Thanks,
Mark.