linux-kernel - Re: [PATCH v1 12/14] x86/boot: tweak a20.c for better code generation

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Message-ID: <981124b5-040e-400b-9912-15a65fdfdfdd@zytor.com>
Date: Fri, 23 Jan 2026 20:24:55 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: "Maciej W. Rozycki" <macro@...am.me.uk>,
        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 12/14] x86/boot: tweak a20.c for better code generation

On 2026-01-23 19:00, Maciej W. Rozycki wrote:
> On Wed, 21 Jan 2026, David Laight wrote:
> 
>> No loops needed.
> 
>  A loop is needed because there can be a considerable delay from issuing 
> the I/O request to flip the A20 gate till the circuitry responding.  This 
> is particularly true with the command issued to the 8042 device, which is 
> a microcontroller running its own firmware that needs it time to process 
> an incoming request to drive one of the microcontroller's GPIOs.  There 
> was a reason for port 0x92 circuitry later added to the PC architecture 
> with the IBM PS/2 being called the "fast A20 gate".
> 

Indeed. I thought I had responded to this already but I hadn't, apparently.

Note that the "long" delay is 2^21 loops! That number wasn't taken out of the
air, either; we found machines that actually needed that many iterations.

In the case where A20 is enabled already, the loop terminates on either the
first or second iteration (the second iteration is when the value at 0x1000200
is exactly 1 higher than the value at 0x200.

Modern machines (Nehalem+) already have A20 enabled, and most machines of the
i686+ generation implement int 0x15 function 0x2401.

	-hpa