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

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Message-ID: <20251216195912.0727cc0d@pumpkin>
Date: Tue, 16 Dec 2025 19:59:12 +0000
From: David Laight <david.laight.linux@...il.com>
To: "H. Peter Anvin" <hpa@...or.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 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 :)
>