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

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Message-ID: <20221118140951.GE4046@willie-the-truck>
Date:   Fri, 18 Nov 2022 14:09:52 +0000
From:   Will Deacon <will@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
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 01:57:52PM +0000, Mark Rutland wrote:
> 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:
> > > > > +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 ?

Nah, it's ok, mainly just me learning what this is doing and I've queued it
locally now.

Will