Re: [Xen-devel] Fix off-by-one comparison when using apic_flat mode

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.03.12 at 10:20, Andrew Jones <drjones@xxxxxxxxxx> wrote:

First of all, assuming this is intended for the 2.6.18 tree, please in
the future indicate so in the subject.

> This patch fixes an off-by-one error in the genapic code so that apic_flat
> is only used when the maximum APIC ID is less than 8, not also 8 (9 IDs).
> 
> Without this patch the kernel may hang when it attempts to process 
> interrupts,
> if there are exactly 9 cpus assigned.
> 
> Signed-off-by: Andrew Jones <drjones@xxxxxxxxxx>
> ---
>  arch/x86_64/kernel/genapic-xen.c |  2 +-
>  arch/x86_64/kernel/genapic.c     |  2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86_64/kernel/genapic-xen.c 
> b/arch/x86_64/kernel/genapic-xen.c
> --- a/arch/x86_64/kernel/genapic-xen.c
> +++ b/arch/x86_64/kernel/genapic-xen.c
> @@ -85,7 +85,7 @@
>                  we have ACPI platform support for CPU hotplug
>                  we should detect hotplug capablity from ACPI tables and
>                  only do this when really needed. -AK */
> -             if (max_apic <= 8)
> +             if (max_apic < 8)

This is inside an #ifndef CONFIG_XEN block, and hence benign.

>                       genapic = &apic_flat;
>  #endif
>               goto print;
> diff --git a/arch/x86_64/kernel/genapic.c b/arch/x86_64/kernel/genapic.c
> --- a/arch/x86_64/kernel/genapic.c
> +++ b/arch/x86_64/kernel/genapic.c
> @@ -79,7 +79,7 @@
>                  we have ACPI platform support for CPU hotplug
>                  we should detect hotplug capablity from ACPI tables and
>                  only do this when really needed. -AK */
> -             if (max_apic <= 8)
> +             if (max_apic < 8)

And this is for native code, so irrelevant for the Xen tree.

>                       genapic = &apic_flat;
>  #endif
>               goto print;

Please clarify your intentions with this patch.

Jan


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