Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 14 Oct 2022 19:41:29 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     David Laight <David.Laight@ACULAB.COM>,
        "'Xin Li'" <xin3.li@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>
CC:     "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "brgerst@gmail.com" <brgerst@gmail.com>,
        "chang.seok.bae@intel.com" <chang.seok.bae@intel.com>
Subject: RE: [PATCH v3 3/6] x86/gsseg: make asm_load_gs_index() take an u16
User-Agent: K-9 Mail for Android
In-Reply-To: <86f60a8edf0e458c86cd84970364cfa2@AcuMS.aculab.com>
References: <20221013200134.1487-1-xin3.li@intel.com> <20221013200134.1487-4-xin3.li@intel.com> <86f60a8edf0e458c86cd84970364cfa2@AcuMS.aculab.com>
Message-ID: <1FCB6543-A666-4423-986B-4CDA1B4DD016@zytor.com>
MIME-Version: 1.0
Content-Type: text/plain;
 charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On October 14, 2022 5:28:25 AM PDT, David Laight <David=2ELaight@ACULAB=2EC=
OM> wrote:
>From: Xin Li
>> Sent: 13 October 2022 21:02
>>=20
>> From: "H=2E Peter Anvin (Intel)" <hpa@zytor=2Ecom>
>>=20
>> Let gcc know that only the low 16 bits of load_gs_index() argument
>> actually matter=2E It might allow it to create slightly better
>> code=2E However, do not propagate this into the prototypes of functions
>> that end up being paravirtualized, to avoid unnecessary changes=2E
>
>Using u16 will almost always make the code worse=2E
>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=2E
>
>	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=2E