[tip:x86/urgent] x86/topology: Handle CPUID bogosity gracefully

[tip-bot for Thomas Gleixner]

Commit-ID:  56402d63eefe22179f7311a51ff2094731420406
Gitweb:     http://git.kernel.org/tip/56402d63eefe22179f7311a51ff2094731420406
Author:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Fri, 6 May 2016 20:48:16 +0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Sat, 7 May 2016 10:06:55 +0200

x86/topology: Handle CPUID bogosity gracefully

Joseph reported that a XEN guest dies with a division by 0 in the package
topology setup code. This happens if cpu_info.x86_max_cores is zero.

Handle that case and emit a warning. This does not fix the underlying XEN bug,
but makes the code more robust.

Reported-and-tested-by: Joseph Salisbury <joseph.salisbury@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
Link: alpine.DEB.2.11.1605062046270.3540@nanos">http://lkml.kernel.org/r/alpine.DEB.2.11.1605062046270.3540@nanos
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 arch/x86/kernel/smpboot.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index a2065d3..0e4329e 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -332,6 +332,11 @@ static void __init smp_init_package_map(void)
 	 * primary cores.
 	 */
 	ncpus = boot_cpu_data.x86_max_cores;
+	if (!ncpus) {
+		pr_warn("x86_max_cores == zero !?!?");
+		ncpus = 1;
+	}
+
 	__max_logical_packages = DIV_ROUND_UP(total_cpus, ncpus);
 
 	/*