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

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Message-ID: <589429e0-e8c8-4949-972b-42d5bbfd83a3@zytor.com>
Date: Tue, 20 Jan 2026 16:47:28 -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.
> 

To address your specific question, no I don't think that is a good idea, since
it would imply that you can reach an arbitrary linear address, which is
definitely not the case.

At least this way the user has to look at what they are doing, which I think
is a good thing for this specific code.

	-hpa