linux-kernel - Re: [PATCH] x86: add hintable NOPs emulation

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Message-ID: <666f534d-b974-4f39-9356-3e2f1ab178f0@orca.pet>
Date: Thu, 21 Aug 2025 15:45:29 +0200
From: Marcos Del Sol Vives <marcos@...a.pet>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>,
 Uros Bizjak <ubizjak@...il.com>, Ard Biesheuvel <ardb@...nel.org>,
 David Kaplan <david.kaplan@....com>, "Ahmed S. Darwish"
 <darwi@...utronix.de>, Kees Cook <kees@...nel.org>,
 Andrew Cooper <andrew.cooper3@...rix.com>, Oleg Nesterov <oleg@...hat.com>,
 "Xin Li (Intel)" <xin@...or.com>, Sabyrzhan Tasbolatov <snovitoll@...il.com>
Subject: Re: [PATCH] x86: add hintable NOPs emulation

El 21/08/2025 a las 14:48, Peter Zijlstra escribió:
> On Wed, Aug 20, 2025 at 03:34:46AM +0200, Marcos Del Sol Vives wrote:
>> +	/* 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.
> 

I can add it no problem for clarity, but would it be really necessary?

All opcodes in that range will have that length by the Intel SDM, so it seems
somewhat redundant, and if the opcode couldn't be read in full the decode
would've failed earlier.

insn_decode_mmio for example which I used as an example of software parsing
of instructions does not check any length if the prefix was 0x0f.