linux-kernel - Re: [PATCH 0/5] x86: Cleanups around slow_down

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Message-ID: <69AE77E6-4256-4B0B-970E-194B4C70B7AF@zytor.com>
Date: Tue, 16 Dec 2025 13:50:42 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: David Laight <david.laight.linux@...il.com>
CC: Jürgen Groß <jgross@...e.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, virtualization@...ts.linux.dev, kvm@...r.kernel.org,
        linux-hwmon@...r.kernel.org, linux-block@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ajay Kaher <ajay.kaher@...adcom.com>,
        Alexey Makhalov <alexey.makhalov@...adcom.com>,
        Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org, Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>, Denis Efremov <efremov@...ux.com>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 0/5] x86: Cleanups around slow_down_io()

On December 16, 2025 11:59:12 AM PST, David Laight <david.laight.linux@...il.com> wrote:
>On Tue, 16 Dec 2025 07:32:09 -0800
>"H. Peter Anvin" <hpa@...or.com> wrote:
>
>> On December 16, 2025 5:55:54 AM PST, "Jürgen Groß" <jgross@...e.com> wrote:
>> >On 16.12.25 14:48, Ingo Molnar wrote:  
>> >> 
>> >> * Jürgen Groß <jgross@...e.com> wrote:
>> >>   
>> >>>> CPUs anymore. Should it cause any regressions, it's easy to bisect to.
>> >>>> There's been enough changes around all these facilities that the
>> >>>> original timings are probably way off already, so we've just been
>> >>>> cargo-cult porting these to newer kernels essentially.  
>> >>> 
>> >>> Fine with me.
>> >>> 
>> >>> Which path to removal of io_delay would you (and others) prefer?
>> >>> 
>> >>> 1. Ripping it out immediately.  
>> >> 
>> >> I'd just rip it out immediately, and see who complains. :-)  
>> >
>> >I figured this might be a little bit too evil. :-)
>> >
>> >I've just sent V2 defaulting to have no delay, so anyone hit by that
>> >can still fix it by applying the "io_delay" boot parameter.
>> >
>> >I'll do the ripping out for kernel 6.21 (or whatever it will be called).
>> >
>> >
>> >Juergen  
>> 
>> Ok, I'm going to veto ripping it out from the real-mode init code,
>> because I actually know why it is there :) ...
>
>Pray tell.
>One thing I can think of is the delay allows time for a level-sensitive
>IRQ line to de-assert before an ISR exits.
>Or, maybe more obscure, to avoid back to back accesses to some register
>breaking the 'inter-cycle recovery time' for the device.
>That was a good way to 'break' the Zilog SCC and the 8259 interrupt
>controller (eg on any reference board with a '286 cpu).
>
>	David
>
>> and that code is pre-UEFI legacy these days anyway.
>> 
>> Other places... I don't care :)
>> 
>
>

A20 gate logic on some motherboards, especially.