[PATCH 49/50] intel idle: Make idle driver more robust

[Len Brown]

From: Thomas Renninger <trenn@xxxxxxx>

kvm -cpu host passes the original cpuid info to the guest.

Latest kvm version seem to return true for mwait_leaf cpuid
function on recent Intel CPUs. But it does not return mwait
C-states (mwait_substates), instead zero is returned.

While real CPUs seem to always return non-zero values, the intel
idle driver should not get active in kvm (mwait_substates == 0)
case and bail out.
Otherwise a Null pointer exception will happen later when the
cpuidle subsystem tries to get active:
[0.984807] BUG: unable to handle kernel NULL pointer dereference at (null)
[0.984807] IP: [<(null)>] (null)
...
[0.984807][<ffffffff8143cf34>] ? cpuidle_idle_call+0xb4/0x340
[0.984807][<ffffffff8159e7bc>] ? __atomic_notifier_call_chain+0x4c/0x70
[0.984807][<ffffffff81001198>] ? cpu_idle+0x78/0xd0

Reference:
https://bugzilla.novell.com/show_bug.cgi?id=726296

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Thomas Renninger <trenn@xxxxxxx>
CC: Bruno Friedmann <bruno@xxxxxxxxxxx>
Signed-off-by: Len Brown <len.brown@xxxxxxxxx>
---
 drivers/idle/intel_idle.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index 6a3d91d..e56ae0b 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -356,7 +356,8 @@ static int intel_idle_probe(void)
 	cpuid(CPUID_MWAIT_LEAF, &eax, &ebx, &ecx, &mwait_substates);
 
 	if (!(ecx & CPUID5_ECX_EXTENSIONS_SUPPORTED) ||
-		!(ecx & CPUID5_ECX_INTERRUPT_BREAK))
+	    !(ecx & CPUID5_ECX_INTERRUPT_BREAK) ||
+	    !mwait_substates)
 			return -ENODEV;
 
 	pr_debug(PREFIX "MWAIT substates: 0x%x\n", mwait_substates);
-- 
1.7.9.rc1

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