linux-kernel - Re: [PATCH v2 02/14] x86/alternatives: Optimize optimize

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Date:   Mon, 22 Mar 2021 09:17:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, jpoimboe@...hat.com, jgross@...e.com,
        mbenes@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/14] x86/alternatives: Optimize optimize_nops()

On Sun, Mar 21, 2021 at 01:06:47PM +0100, Borislav Petkov wrote:

> I think I've made this simpler; pasting the whole function and not the
> diff because former is easier to read:

You've make it only replace a single stream of NOPs. Which is probably
fine, but... :-)

So mine, while a little more complicated, will replace any number of NOP
streams in any location. Saves having to worry about if we missed some
stupid corner case somewhere.

Anyway, I think I got it slightly wrong.. How's this one?

---

static void __init_or_module noinline optimize_nops(struct alt_instr *a, u8 *instr)
{
	int nops = 0, i = 0;
	bool done = false;
	struct insn insn;
	u8 *nop = NULL;

	do {
		if (insn_decode_kernel(&insn, &instr[i]))
			return;

		if (insn.length == 1 && insn.opcode.bytes[0] == 0x90)
			nops++;
		else if (nops)
			nop = &instr[i - nops];

		if (i + insn.length >= a->instrlen) {
			nop = &instr[i - nops];
			done = true;
		}

		if (nop) {
			unsigned long flags;

			local_irq_save(flags);
			add_nops(nop, nops);
			local_irq_restore(flags);

			DUMP_BYTES(instr, a->instrlen, "%px: [%d:%d) optimized NOPs: ",
				   instr, (int)(unsigned long)(nop-instr), nops);

			nops = 0;
			nop = NULL;
		}

		i += insn.length;

	} while (!done);

	WARN_ON_ONCE(nop);
}