Re: [PATCH] x86: add hintable NOPs emulation

[H. Peter Anvin]

On August 21, 2025 5:48:04 AM PDT, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>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;
>> +}

No, hintable noops apply to any modr/m.