linux-kernel - Re: 4.8.2 not booting in 32-bit VM without I/O-APIC

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Date:   Fri, 28 Oct 2016 21:34:53 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Michal Necasek <michal.necasek@...cle.com>
cc:     prarit@...hat.com, ville.syrjala@...ux.intel.com,
        michael.thayer@...cle.com, knut.osmundsen@...cle.com,
        frank.mehnert@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: 4.8.2 not booting in 32-bit VM without I/O-APIC

On Fri, 28 Oct 2016, Thomas Gleixner wrote:

> On Fri, 28 Oct 2016, Michal Necasek wrote:
> > 
> >  Sorry if I wasn't clear. No, it doesn't work for us. The ff8560512b8d
> > commit fixes the APIC poking on machines where there is no local APIC
> > available (and thus fixes the committer's machine), but it doesn't work
> > in our case, where there is a local APIC but no I/O APIC, no ACPI MADT,
> > and no MP tables.
> 
> > In other words, for us boot_cpu_has(X86_FEATURE_APIC) is true but the
> > APIC has not been set up yet because init_apic_mappings() hasn't been
> > run.

Right. That mapping setup is an utter trainwreck as we do it from multiple
places, but there is no reason why we can't move it before the call to
prefill_possible_map().

Thanks,

	tglx

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index bbfbca5fea0c..b59fdba3cbdf 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1221,11 +1221,13 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	get_smp_config();
 
+	/* Make sure apic is mapped before prefill_possible_map() */
+	init_apic_mappings();
+
 	prefill_possible_map();
 
 	init_cpu_to_node();
 
-	init_apic_mappings();
 	io_apic_init_mappings();
 
 	kvm_guest_init();