linux-kernel - RE: arch/x86/kernel/apic/apic.c: calibrate_APIC_clock() soft hangs when PIC is not configured by BIOS before kernel is launched.

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Date:   Fri, 14 Jun 2019 15:02:43 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirkendall, Garrett" <Garrett.Kirkendall@....com>
cc:     "nstange@...e.de" <nstange@...e.de>,
        "luto@...nel.org" <luto@...nel.org>,
        "natechancellor@...il.com" <natechancellor@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: arch/x86/kernel/apic/apic.c: calibrate_APIC_clock() soft hangs
 when PIC is not configured by BIOS before kernel is launched.

Garrett,

On Thu, 9 May 2019, Kirkendall, Garrett wrote:

  A: Because it messes up the order in which people normally read text.
  Q: Why is top-posting such a bad thing?
  A: Top-posting.
  Q: What is the most annoying thing in e-mail?

  A: No.
  Q: Should I include quotations after my reply?

  http://daringfireball.net/2007/07/on_top

Also please break the lines around 78 chars.

> 1.  Is it correct to probe the 8259 before it is initialized by the
>     kernel?  The 8259 will not respond properly to the probe unless it is
>     properly initialized.

So far the kernel relied on the BIOS to initialize 8259

> 2.  Should IOAPIC interrupts 0-15 require the legacy PIC be available and
>     initialized by the BIOS?

Unless the platform explicitely states that there is no legacy PIC, which
is true for some MID and HV guest systems. 

> 2.  The kernel will not boot if there is no legacy 8259 PIC even if all
>     the other factors stated are provided.

Hmm? what other factors?

> I want to understand why a preinitialized 8259 is a requirement for a
> system configured to use the IOAPIC?

Mostly historical reasons and the whole PIC/IOAPIC thing has been a fragile
nightmare forever, so I'm reluctant to do any extra work there which might
break older machines.

Thanks,

	tglx