linux-kernel - Re: [PATCH v3 3/6] x86/static_call: Add out-of-line static call implementation

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Message-ID: <20190110162810.fgrmezgecvbekloe@treble>
Date:   Thu, 10 Jan 2019 10:28:10 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nadav Amit <namit@...are.com>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v3 3/6] x86/static_call: Add out-of-line static call
 implementation

On Thu, Jan 10, 2019 at 12:16:34AM +0000, Nadav Amit wrote:
> > On Jan 9, 2019, at 2:59 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > 
> 
> <snip>
> 
> > +
> > +void __ref arch_static_call_transform(void *site, void *tramp, void *func)
> > +{
> > +	s32 dest_relative;
> > +	unsigned char opcode;
> > +	void *(*poker)(void *, const void *, size_t);
> > +	void *insn = tramp;
> > +
> > +	mutex_lock(&text_mutex);
> > +
> > +	/*
> > +	 * For x86-64, a 32-bit cross-modifying write to a call destination is
> > +	 * safe as long as it's within a cache line.
> > +	 */
> > +	opcode = *(unsigned char *)insn;
> > +	if (opcode != 0xe8 && opcode != 0xe9) {
> > +		WARN_ONCE(1, "unexpected static call insn opcode 0x%x at %pS",
> > +			  opcode, insn);
> > +		goto done;
> > +	}
> > +
> > +	dest_relative = (long)(func) - (long)(insn + CALL_INSN_SIZE);
> > +
> > +	poker = early_boot_irqs_disabled ? text_poke_early : text_poke;
> > +	poker(insn + 1, &dest_relative, sizeof(dest_relative));
> > +
> > +done:
> > +	mutex_unlock(&text_mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(arch_static_call_transform);
> 
> Err… I was rewriting __jump_label_transform(), so if this code duplication can
> be avoided, this would be great.
> 
> (See https://lkml.org/lkml/2018/11/14/72 )

I don't see much code duplication, because __jump_label_transform() uses
text_poke_bp(), whereas this uses text_poke().

It's true they both fall back to text_poke_early(), but I don't see any
opportunities for sharing code there, unless we decide to go back to
using breakpoints.

-- 
Josh