Re: [PATCH v2 1/6] KVM: s390: interrupt: Fix single-stepping into interrupt handlers

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On 21.07.23 13:57, Ilya Leoshkevich wrote:
After single-stepping an instruction that generates an interrupt, GDB
ends up on the second instruction of the respective interrupt handler.

The reason is that vcpu_pre_run() manually delivers the interrupt, and
then __vcpu_run() runs the first handler instruction using the
CPUSTAT_P flag. This causes a KVM_SINGLESTEP exit on the second handler
instruction.

Fix by delaying the KVM_SINGLESTEP exit until after the manual
interrupt delivery.

Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
---
  arch/s390/kvm/interrupt.c | 10 ++++++++++
  arch/s390/kvm/kvm-s390.c  |  4 ++--
  2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 9bd0a873f3b1..2cebe4227b8e 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -1392,6 +1392,7 @@ int __must_check kvm_s390_deliver_pending_interrupts(struct kvm_vcpu *vcpu)
  {
  	struct kvm_s390_local_interrupt *li = &vcpu->arch.local_int;
  	int rc = 0;
+	bool delivered = false;
  	unsigned long irq_type;
  	unsigned long irqs;
@@ -1465,6 +1466,15 @@ int __must_check kvm_s390_deliver_pending_interrupts(struct kvm_vcpu *vcpu)
  			WARN_ONCE(1, "Unknown pending irq type %ld", irq_type);
  			clear_bit(irq_type, &li->pending_irqs);
  		}
+		delivered |= !rc;
+	}
+


Can we add a comment like

/*
 * We delivered at least one interrupt and modified the PC. Force a
 * singlestep event now.
 */

+	if (delivered && guestdbg_sstep_enabled(vcpu)) {
+		struct kvm_debug_exit_arch *debug_exit = &vcpu->run->debug.arch;
+
+		debug_exit->addr = vcpu->arch.sie_block->gpsw.addr;
+		debug_exit->type = KVM_SINGLESTEP;
+		vcpu->guest_debug |= KVM_GUESTDBG_EXIT_PENDING;
  	}

I do wonder if we, instead, want to do this whenever we modify the PSW.

That way we could catch any PC changes and only have to add checks for guestdbg_exit_pending().


But this is simpler and should work as well.

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb




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