KVM: s390: interrupt: Fix single-stepping userspace-emulated instructions

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().

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-ID: <20230725143857.228626-5-iii@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 0c6333b..e651160 100644 (file)
--- 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,