linux-kernel - Re: [PATCH v1 11/14] x86/boot: use __seg_fs and __seg

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Message-ID: <CAFULd4ay9rcjx_5V5E+f7wCcUQpei85iOwwLrwgYgCmbFiaXWw@mail.gmail.com>
Date: Wed, 21 Jan 2026 17:03:56 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.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 11/14] x86/boot: use __seg_fs and __seg_gs in the
 real-mode boot code

On Wed, Jan 21, 2026 at 4:14 PM H. Peter Anvin <hpa@...or.com> wrote:

> >My proposal is to use a volatile pointer to an absolute address, so
> >the unwanted optimizations are suppressed. The generated code is the
> >same as with barrier(), but now the code tells the compiler that every
> >read and write to this address must happen exactly as written in the
> >source code. Before your patch, the accessors were defined with
> >volatile asm, and this is the place where volatile qualifier matters.
> >So, my proposed code would read:
> >
> >#define A20_TEST_ADDR    (4*0x80)
> >
> >#define A20_TEST_GS (*(volatile __seg_gs u32 *)A20_TEST_ADDR)
> >#define A20_TEST_FS (*(volatile __seg_fs u32 *)(A20_TEST_ADDR+0x10))
> >
> >static int a20_test(int loops)
> >{
> >    int saved, ctr;
> >
> >    set_fs(0xffff);
> >
> >    saved = ctr = A20_TEST_GS;
> >
> >    do {
> >        A20_TEST_GS = ++ctr;
> >        io_delay();    /* Make constant delay */
> >        if (A20_TEST_FS != ctr)
> >            break;
> >    } while (--loops);
> >
> >    A20_TEST_GS = saved;
> >    return loops;
> >}
> >
> >BR,
> >Uros.
> >
>
> I disagree with that being the preferred solution, and it isn't really the Linux style.
>
> Not only does it require making more changes to the macros, but the barrier() construct is well established in Linux as the way to indicate that a memory variable is subject to examination and/or modification by another system agent, while still being a memory variable.
>
> It also generally produces better code.
>
> So no, your analysis is, in my opinion, incorrect in light of the way the Linux memory model is already used.

Thanks for explaining to me the Linux way! If this is the case, I will
withdraw my proposed solution.

Best regards,
Uros.