linux-kernel - Re: [PATCHv2 1/8] ftrace: add ftrace_init

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Message-ID: <20191106141530.GC50610@lakrids.cambridge.arm.com>
Date:   Wed, 6 Nov 2019 14:15:30 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Amit Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, catalin.marinas@....com,
        deller@....de, duwe@...e.de, James.Bottomley@...senPartnership.com,
        james.morse@....com, jeyu@...nel.org, jpoimboe@...hat.com,
        jthierry@...hat.com, linux-parisc@...r.kernel.org,
        mingo@...hat.com, peterz@...radead.org, svens@...ckframe.org,
        takahiro.akashi@...aro.org, will@...nel.org
Subject: Re: [PATCHv2 1/8] ftrace: add ftrace_init_nop()

On Tue, Nov 05, 2019 at 12:17:26PM +0530, Amit Kachhap wrote:
> On 11/4/19 7:06 PM, Mark Rutland wrote:
> > On Sat, Nov 02, 2019 at 05:49:00PM +0530, Amit Daniel Kachhap wrote:
> > > On 10/29/19 10:28 PM, Mark Rutland wrote:
> > > > +/**
> > > > + * ftrace_init_nop - initialize a nop call site
> > > > + * @mod: module structure if called by module load initialization
> > > > + * @rec: the call site record (e.g. mcount/fentry)
> > > > + *
> > > > + * This is a very sensitive operation and great care needs
> > > > + * to be taken by the arch.  The operation should carefully
> > > > + * read the location, check to see if what is read is indeed
> > > > + * what we expect it to be, and then on success of the compare,
> > > > + * it should write to the location.
> > > > + *
> > > > + * The code segment at @rec->ip should contain the contents created by
> > > > + * the compiler
> > > Nit: Will it be better to write it as "@rec->ip should store the adjusted
> > > ftrace entry address of the call site" or something like that.
> > 
> > This was the specific wording requested by Steve, and it's trying to
> > describe the instructions at rec->ip, rather than the value of rec->ip,
> > so I think it's better to leave this as-is.
> ok Its fine this way too. Actually from the comment, I could not understand
> which one of the compiler contents this points to as in this case there are
> 2 nops.

We can't say what the compiler contents will be. An architecture may use
this callback if it's using mcount, mfentry, patchable-function-entry,
or some other mechanism we're not aware of today. Depending on the
architecture and mechanism, the callsite could contain a number of
distinct things.

All the comment is trying to say is that when ftrace_init_nop() is
called, the callsite has not been modified in any way since being
compiled, so we can expect the contents to be whatever the compiler
generated.

Thanks,
Mark.