[PATCH 3/3] KVM: PPC: Book3S HV: Clear pending decr exceptions on nested guest entry

[Suraj Jitindar Singh <sjitindarsingh@gmail.com>]

If we enter an L1 guest with a pending decrementer exception then this
is cleared on guest exit if the guest has writtien a positive value into
the decrementer (indicating that it handled the decrementer exception)
since there is no other way to detect that the guest has handled the
pending exception and that it should be dequeued. In the event that the
L1 guest tries to run a nested (L2) guest immediately after this and the
L2 guest decrementer is negative (which is loaded by L1 before making
the H_ENTER_NESTED hcall), then the pending decrementer exception
isn't cleared and the L2 entry is blocked since L1 has a pending
exception, even though L1 may have already handled the exception and
written a positive value for it's decrementer. This results in a loop of
L1 trying to enter the L2 guest and L0 blocking the entry since L1 has
an interrupt pending with the outcome being that L2 never gets to run
and hangs.

Fix this by clearing any pending decrementer exceptions when L1 makes
the H_ENTER_NESTED hcall since it won't do this if it's decrementer has
gone negative, and anyway it's decrementer has been communicated to L0
in the hdec_expires field and L0 will return control to L1 when this
goes negative by delivering an H_DECREMENTER exception.

Fixes: 95a6432ce903 "KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests"

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
---
 arch/powerpc/kvm/book3s_hv.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 719fd2529eec..4a5eb29b952f 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -4128,8 +4128,15 @@ int kvmhv_run_single_vcpu(struct kvm_run *kvm_run,
 
 	preempt_enable();
 
-	/* cancel pending decrementer exception if DEC is now positive */
-	if (get_tb() < vcpu->arch.dec_expires && kvmppc_core_pending_dec(vcpu))
+	/*
+	 * cancel pending decrementer exception if DEC is now positive, or if
+	 * entering a nested guest in which case the decrementer is now owned
+	 * by L2 and the L1 decrementer is provided in hdec_expires
+	 */
+	if (kvmppc_core_pending_dec(vcpu) &&
+			((get_tb() < vcpu->arch.dec_expires) ||
+			 (trap == BOOK3S_INTERRUPT_SYSCALL &&
+			  kvmppc_get_gpr(vcpu, 3) == H_ENTER_NESTED)))
 		kvmppc_core_dequeue_dec(vcpu);
 
 	trace_kvm_guest_exit(vcpu);
-- 
2.13.6