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

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 1 of 7] APIC BUG: fix potential Protection Fault during shutdown
From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Date: Tue, 14 Jun 2011 10:44:17 +0100
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On 14/06/11 09:44, Jan Beulich wrote:
>>>> 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
> disabled.
> Jan
As I said, it is an edge case and shouldn't occur under any normal
circumstances, but given the nature of the fix, we might as well help
the odd setups.  I considered upstreaming it to Linux but I doubt It
will be taken because there is no way to force their code to have a
protection fault.

>> 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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>> http://lists.xensource.com/xen-devel 

Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com

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