linux-kernel - Re: [PATCH v6 10/17] x86/static_call: Add inline static call implementation for x86-64

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Message-ID: <20200711095639.GA597537@hirez.programming.kicks-ass.net>
Date:   Sat, 11 Jul 2020 11:56:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com,
        pbonzini@...hat.com, mathieu.desnoyers@...icios.com,
        linux@...musvillemoes.dk
Subject: Re: [PATCH v6 10/17] x86/static_call: Add inline static call
 implementation for x86-64

On Fri, Jul 10, 2020 at 06:31:55PM -0400, Steven Rostedt wrote:
> On Fri, 10 Jul 2020 15:38:41 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:

> > +		/* find key symbol */
> > +		key_name = strdup(insn->call_dest->name);
> 
> Should check for failed allocation of strdup().

Rigt, I'll go add the perror and exit.

> > +		if (strncmp(key_name, STATIC_CALL_TRAMP_PREFIX_STR,
> > +			    strlen(STATIC_CALL_TRAMP_PREFIX_STR))) {
> > +			WARN("static_call: trampoline name malformed: %s", key_name);
> > +			return -1;
> > +		}
> > +		tmp = key_name + STATIC_CALL_TRAMP_PREFIX_LEN - STATIC_CALL_KEY_PREFIX_LEN;
> > +		memcpy(tmp, STATIC_CALL_KEY_PREFIX_STR, STATIC_CALL_KEY_PREFIX_LEN);
> > +
> > +		key_sym = find_symbol_by_name(file->elf, tmp);
> > +		if (!key_sym) {
> > +			WARN("static_call: can't find static_call_key symbol: %s", tmp);
> > +			return -1;
> > +		}
> > +		free(key_name);
> > +
> > +		/* populate reloc for 'key' */
> > +		reloc = malloc(sizeof(*reloc));
> > +		if (!reloc) {
> > +			perror("malloc");
> > +			return -1;
> > +		}
> > +		memset(reloc, 0, sizeof(*reloc));
> > +		reloc->sym = key_sym;
> > +		reloc->addend = 0;
> > +		reloc->type = R_X86_64_PC32;
> 
> How easy would this be for other architectures to implement this? That
> is how much of this function is x86 specific?

I'm not sure; I think it's fairly generic except for the reloc types. So
ARM64 for example might need to use SHT_REL and R_AARCH64_PREL32
instead. But I suppose we'll see that if/when someone tries to make it
work.