linux-kernel - Re: [PATCH RFC] x86/acpi: don't ignore I/O APICs just because there's no local APIC

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Message-ID: <20090612201117.GA29923@lenovo>
Date:	Sat, 13 Jun 2009 00:11:17 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH RFC] x86/acpi: don't ignore I/O APICs just because
	there's no local APIC

[Jeremy Fitzhardinge - Fri, Jun 12, 2009 at 11:22:48AM -0700]
> Parse the ACPI MADT for I/O APIC information, even if the cpu has no
> (apparent) local APIC (ie, the CPU's APIC feature flag is clear).
>
> In principle, the local APIC and the I/O APIC are distinct (but related)
> components, which can be independently present.
>
> In practice this can happen in a Xen system, where the hypervisor has
> full control over the local APICs, and delivers interrupts initiated by
> the I/O APICs via Xen's event channel mechanism.
>
> (This eliminates the need for any explicit if (xen...) tests in
> acpi/boot.c)
>
> Signed-off-by: Jeremy Fitzhardinge<jeremy.fitzhardinge@...rix.com>
>
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index 2410469..19d13e5 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -193,9 +193,6 @@ static int __init acpi_parse_madt(struct acpi_table_header *table)
>  {
>  	struct acpi_table_madt *madt = NULL;
>
> -	if (!cpu_has_apic)
> -		return -EINVAL;
> -

Hi Jeremy,

just for the record -- this removement has a side effect.
Imagine I've passed "disableapic" so I expect as many as
possible apic-related code would not pass thru execution.
Now we would have (say for IBM Summit)

acpi_parse_madt
  default_acpi_madt_oem_check
    summit_acpi_madt_oem_check
      mark_tsc_unstable
      setup_summit

Dunno if it harmless or no but it changes kernel behaviour.
cpu_has_apic cleared if disableapic option (which is early
param) passed to kernel.

Just a note. Didn't walk thru all ways.

	-- Cyrill
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