Re: [PATCH v3 4/6] KVM: s390: interrupt: Fix single-stepping userspace-emulated instructions

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On 24.07.23 11:44, Ilya Leoshkevich wrote:
Single-stepping a userspace-emulated instruction that generates an
interrupt causes GDB to land on the instruction following it instead of
the respective interrupt handler.

The reason is that after arranging a KVM_EXIT_S390_SIEIC exit,
kvm_handle_sie_intercept() calls kvm_s390_handle_per_ifetch_icpt(),
which sets KVM_GUESTDBG_EXIT_PENDING. This bit, however, is not
processed immediately, but rather persists until the next ioctl(),
causing a spurious single-step exit.

Fix by clearing this bit in ioctl().

Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
---
  arch/s390/kvm/kvm-s390.c | 23 ++++++++++++++++++++---
  1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 0c6333b108ba..e6511608280c 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -5383,6 +5383,7 @@ long kvm_arch_vcpu_async_ioctl(struct file *filp,
  {
  	struct kvm_vcpu *vcpu = filp->private_data;
  	void __user *argp = (void __user *)arg;
+	int rc;
switch (ioctl) {
  	case KVM_S390_IRQ: {
@@ -5390,7 +5391,8 @@ long kvm_arch_vcpu_async_ioctl(struct file *filp,
if (copy_from_user(&s390irq, argp, sizeof(s390irq)))
  			return -EFAULT;
-		return kvm_s390_inject_vcpu(vcpu, &s390irq);
+		rc = kvm_s390_inject_vcpu(vcpu, &s390irq);
+		break;
  	}
  	case KVM_S390_INTERRUPT: {
  		struct kvm_s390_interrupt s390int;
@@ -5400,10 +5402,25 @@ long kvm_arch_vcpu_async_ioctl(struct file *filp,
  			return -EFAULT;
  		if (s390int_to_s390irq(&s390int, &s390irq))
  			return -EINVAL;
-		return kvm_s390_inject_vcpu(vcpu, &s390irq);
+		rc = kvm_s390_inject_vcpu(vcpu, &s390irq);
+		break;
  	}
+	default:
+		rc = -ENOIOCTLCMD;
+		break;
  	}
-	return -ENOIOCTLCMD;
+
+	/*
+	 * To simplify single stepping of userspace-emulated instructions,
+	 * KVM_EXIT_S390_SIEIC exit sets KVM_GUESTDBG_EXIT_PENDING (see
+	 * should_handle_per_ifetch()). However, if userspace emulation injects
+	 * an interrupt, it needs to be cleared, so that KVM_EXIT_DEBUG happens
+	 * after (and not before) the interrupt delivery.
+	 */
+	if (!rc)
+		vcpu->guest_debug &= ~KVM_GUESTDBG_EXIT_PENDING;
+
+	return rc;
  }
static int kvm_s390_handle_pv_vcpu_dump(struct kvm_vcpu *vcpu,

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb




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