linux-kernel - RE: [PATCH v3 3/6] x86/gsseg: make asm_load_gs

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Message-ID: <1FCB6543-A666-4423-986B-4CDA1B4DD016@zytor.com>
Date:   Fri, 14 Oct 2022 19:41:29 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "'Xin Li'" <xin3.li@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "brgerst@...il.com" <brgerst@...il.com>,
        "chang.seok.bae@...el.com" <chang.seok.bae@...el.com>
Subject: RE: [PATCH v3 3/6] x86/gsseg: make asm_load_gs_index() take an u16

On October 14, 2022 5:28:25 AM PDT, David Laight <David.Laight@...LAB.COM> wrote:
>From: Xin Li
>> Sent: 13 October 2022 21:02
>> 
>> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
>> 
>> Let gcc know that only the low 16 bits of load_gs_index() argument
>> actually matter. It might allow it to create slightly better
>> code. However, do not propagate this into the prototypes of functions
>> that end up being paravirtualized, to avoid unnecessary changes.
>
>Using u16 will almost always make the code worse.
>At some point the value has to be masked and/or extended
>to ensure an out of range value doesn't appear in
>a register.
>
>	David
>
>-
>Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
>Registration No: 1397386 (Wales)
>
>

Is that a general statement or are you actually invoking it in this case? This is about it being a narrowing input, *removing* such constraints.