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

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Date:   Fri, 14 Oct 2022 12:28:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     '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>,
        "hpa@...or.com" <hpa@...or.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

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

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