[PATCH 4.10 066/129] KVM: PPC: Book3S HV: Dont try to signal cpu -1

[Greg Kroah-Hartman]

4.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Paul Mackerras <paulus@xxxxxxxxxx>

commit 3deda5e50c893be38c1b6b3a73f8f8fb5560baa4 upstream.

If the target vcpu for kvmppc_fast_vcpu_kick_hv() is not running on
any CPU, then we will have vcpu->arch.thread_cpu == -1, and as it
happens, kvmppc_fast_vcpu_kick_hv will call kvmppc_ipi_thread with
-1 as the cpu argument.  Although this is not meaningful, in the past,
before commit 1704a81ccebc ("KVM: PPC: Book3S HV: Use msgsnd for IPIs
to other cores on POWER9", 2016-11-18), it was harmless because CPU
-1 is not in the same core as any real CPU thread.  On a POWER9,
however, we don't do the "same core" check, so we were trying to
do a msgsnd to thread -1, which is invalid.  To avoid this, we add
a check to see that vcpu->arch.thread_cpu is >= 0 before calling
kvmppc_ipi_thread() with it.  Since vcpu->arch.thread_vcpu can change
asynchronously, we use READ_ONCE to ensure that the value we check is
the same value that we use as the argument to kvmppc_ipi_thread().

Fixes: 1704a81ccebc ("KVM: PPC: Book3S HV: Use msgsnd for IPIs to other cores on POWER9")
Signed-off-by: Paul Mackerras <paulus@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/powerpc/kvm/book3s_hv.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -182,7 +182,8 @@ static void kvmppc_fast_vcpu_kick_hv(str
 		++vcpu->stat.halt_wakeup;
 	}
 
-	if (kvmppc_ipi_thread(vcpu->arch.thread_cpu))
+	cpu = READ_ONCE(vcpu->arch.thread_cpu);
+	if (cpu >= 0 && kvmppc_ipi_thread(cpu))
 		return;
 
 	/* CPU points to the first thread of the core */