KVM: PPC: Book3S HV: Handle unexpected traps in guest entry/exit code better

As we saw with the TM Bad Thing type of program interrupt occurring
on the hrfid that enters the guest, it is not completely impossible
to have a trap occurring in the guest entry/exit code, despite the
fact that the code has been written to avoid taking any traps.

This adds a check in the kvmppc_handle_exit_hv() function to detect
the case when a trap has occurred in the hypervisor-mode code, and
instead of treating it just like a trap in guest code, we now print
a message and return to userspace with a KVM_EXIT_INTERNAL_ERROR
exit reason.

Of the various interrupts that get handled in the assembly code in
the guest exit path and that can return directly to the guest, the
only one that can occur when MSR.HV=1 and MSR.EE=0 is machine check
(other than system call, which we can avoid just by not doing a sc
instruction).  Therefore this adds code to the machine check path to
ensure that if the MCE occurred in hypervisor mode, we exit to the
host rather than trying to continue the guest.
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kvm/book3s_hv.c

@@ -827,6 +827,24 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
 
 	vcpu->stat.sum_exits++;
 
+	/*
+	 * This can happen if an interrupt occurs in the last stages
+	 * of guest entry or the first stages of guest exit (i.e. after
+	 * setting paca->kvm_hstate.in_guest to KVM_GUEST_MODE_GUEST_HV
+	 * and before setting it to KVM_GUEST_MODE_HOST_HV).
+	 * That can happen due to a bug, or due to a machine check
+	 * occurring at just the wrong time.
+	 */
+	if (vcpu->arch.shregs.msr & MSR_HV) {
+		printk(KERN_EMERG "KVM trap in HV mode!\n");
+		printk(KERN_EMERG "trap=0x%x | pc=0x%lx | msr=0x%llx\n",
+			vcpu->arch.trap, kvmppc_get_pc(vcpu),
+			vcpu->arch.shregs.msr);
+		kvmppc_dump_regs(vcpu);
+		run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		run->hw.hardware_exit_reason = vcpu->arch.trap;
+		return RESUME_HOST;
+	}
 	run->exit_reason = KVM_EXIT_UNKNOWN;
 	run->ready_for_interrupt_injection = 1;
 	switch (vcpu->arch.trap) {

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -2404,6 +2404,8 @@ machine_check_realmode:
 	 * guest as machine check causing guest to crash.
 	 */
 	ld	r11, VCPU_MSR(r9)
+	rldicl.	r0, r11, 64-MSR_HV_LG, 63 /* check if it happened in HV mode */
+	bne	mc_cont			/* if so, exit to host */
 	andi.	r10, r11, MSR_RI	/* check for unrecoverable exception */
 	beq	1f			/* Deliver a machine check to guest */
 	ld	r10, VCPU_PC(r9)