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Re: [Xen-devel] [PATCH 1 of 7] APIC BUG: fix potential Protection Fault

To: "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 1 of 7] APIC BUG: fix potential Protection Fault during shutdown
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Tue, 14 Jun 2011 09:44:39 +0100
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>>> On 13.06.11 at 19:02, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> This is a rare case, but if the BIOS is set to uniprocessor, and Xen
> is booted with 'lapic x2apic', Xen will switch into x2apic mode, which
> will cause a protection fault when disabling the local APIC.  This
> leads to a general protection fault as this code is also in the fault
> handler.
> When x2apic mode is enabled, the only tranlsation which does
> not result in a protection fault is to clear both the EN and EXTD
> bits, which is safe to do in all cases, even if you are in xapic
> mode rather than x2apic mode.
> The linux code from which this is derrived is protected by an
> if ( ! x2apic_mode ...) clause which is how they get away with it.
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxxxx>

You may want to submit a similar patch to Linux (which is what
this code got derived from), so that in the future no-one will get
surprised that this is different in Xen and Linux.

Otoh, interestingly this is being done only for x86-32 in Linux, and
I highly doubt any X2APIC capable machine would boot with APIC


> diff -r 37c77bacb52a -r 076c3034c8c7 xen/arch/x86/apic.c
> --- a/xen/arch/x86/apic.c     Mon May 23 17:38:28 2011 +0100
> +++ b/xen/arch/x86/apic.c     Mon Jun 13 17:45:43 2011 +0100
> @@ -340,7 +340,8 @@ void disable_local_APIC(void)
>      if (enabled_via_apicbase) {
>          uint64_t msr_content;
>          rdmsrl(MSR_IA32_APICBASE, msr_content);
> -        wrmsrl(MSR_IA32_APICBASE, msr_content & ~MSR_IA32_APICBASE_ENABLE);
> +        wrmsrl(MSR_IA32_APICBASE, msr_content &
>      }
>  }
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