KVM: s390: no timer interrupts when single-stepping a guest

When a guest is single-stepped, we want to disable timer interrupts. Otherwise,
the guest will continuously execute the external interrupt handler and make
debugging of code where timer interrupts are enabled almost impossible.

The delivery of timer interrupts can be enforced in such sections by setting a
breakpoint and continuing execution.

In order to disable timer interrupts, they are disabled in the control register
of the guest just before SIE entry and are suppressed in the interrupt
check/delivery methods.
Signed-off-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>

arch/s390/kvm/guestdbg.c

@@ -155,6 +155,8 @@ void kvm_s390_patch_guest_per_regs(struct kvm_vcpu *vcpu)
 	 */
 
 	if (guestdbg_sstep_enabled(vcpu)) {
+		/* disable timer (clock-comparator) interrupts */
+		vcpu->arch.sie_block->gcr[0] &= ~0x800ul;
 		vcpu->arch.sie_block->gcr[9] |= PER_EVENT_IFETCH;
 		vcpu->arch.sie_block->gcr[10] = 0;
 		vcpu->arch.sie_block->gcr[11] = PSW_ADDR_INSN;

arch/s390/kvm/interrupt.c

@@ -61,6 +61,9 @@ static int ckc_interrupts_enabled(struct kvm_vcpu *vcpu)
 	if (psw_extint_disabled(vcpu) ||
 	    !(vcpu->arch.sie_block->gcr[0] & 0x800ul))
 		return 0;
+	if (guestdbg_enabled(vcpu) && guestdbg_sstep_enabled(vcpu))
+		/* No timer interrupts when single stepping */
+		return 0;
 	return 1;
 }