Re: [PATCH] x86: add hintable NOPs emulation

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Aug 20, 2025 at 03:34:46AM +0200, Marcos Del Sol Vives wrote:
> +static bool handle_hnop(struct pt_regs *regs)
> +{
> +	struct thread_struct *t = &current->thread;
> +	unsigned char buf[MAX_INSN_SIZE];
> +	unsigned long nr_copied;
> +	struct insn insn;
> +
> +	nr_copied = insn_fetch_from_user(regs, buf);
> +	if (nr_copied <= 0)
> +		return false;
> +
> +	if (!insn_decode_from_regs(&insn, regs, buf, nr_copied))
> +		return false;
> +
> +	/* Hintable NOPs cover 0F 18 to 0F 1F */
> +	if (insn.opcode.bytes[0] != 0x0F ||
> +		insn.opcode.bytes[1] < 0x18 || insn.opcode.bytes[1] > 0x1F)
> +		return false;

FWIW, you need to check for insn.opcode.nbytes == 2.

> +	if (!t->hnop_warn) {
> +		pr_warn_ratelimited("%s[%d] emulating hintable NOP, ip:%lx\n",
> +		       current->comm, task_pid_nr(current), regs->ip);
> +		t->hnop_warn = 1;
> +	}
> +
> +	regs->ip += insn.length;
> +	return true;
> +}