>>> 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
> 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 &
> + ~(MSR_IA32_APICBASE_ENABLE|MSR_IA32_APICBASE_EXTD));
> }
> }
>
>
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