linux-kernel - Re: [PATCH v6 5/5] x86/gsseg: use the LKGS instruction if available for load_gs

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Message-ID: <20240701060841.GAZoJH6XpJz6qyo-Lr@fat_crate.local>
Date: Mon, 1 Jul 2024 08:08:41 +0200
From: Borislav Petkov <bp@...en8.de>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Xin Li <xin3.li@...el.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	dave.hansen@...ux.intel.com, peterz@...radead.org,
	brgerst@...il.com, chang.seok.bae@...el.com, jgross@...e.com
Subject: Re: [PATCH v6 5/5] x86/gsseg: use the LKGS instruction if available
 for load_gs_index()

On Sun, Jun 30, 2024 at 07:34:42PM -0700, H. Peter Anvin wrote:
> It is much like "movl %edi,%ds"...

You mean ...,%gs"... :-P

But yeah, that makes more sense.

Are we saying somewhere that when moving into a segreg, only the lower 16 bits
of the source operand are used?

Both docs say

8E /r	MOV Sreg, r/m16		Move r/m16 to segment register.

and I read that strict as r16 so I'd expect a "mov %di,..." not a "mov
%edi,..."

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette