linux-kernel - Re: [PATCH v1 02/14] x86/realmode: make %gs == 0 an invariant

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Message-ID: <dec1687a-a70d-48cb-806b-0320de7f96f1@zytor.com>
Date: Tue, 20 Jan 2026 14:51:50 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Uros Bizjak <ubizjak@...il.com>, Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <kees@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Kiryl Shutsemau <kas@...nel.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev,
        x86@...nel.org
Subject: Re: [PATCH v1 02/14] x86/realmode: make %gs == 0 an invariant

On 2026-01-20 14:40, David Laight wrote:
> 
> Would it be better to wrap that as (say) read_abs_32() since the
> objective is to read an absolute address (using the relevant zero segment
> register) rather than to read though either fs or gs.
> 
> Is this code all running in 32bit linear mode with non-zero cs/ss/ds
> segment registers and 'suitable' entries in the GDT?
> That mode makes my brain hurt :-)
> Or is there a fudge to get 16bit asm from the C.
> 

The code is running in 16-bit real mode, compiled using -m16.

	-hpa