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

David Hildenbrand
2023-07-24T11:34:10Z

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@linux.ibm.com>
> ---
> 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@redhat.com>

--
Cheers,

David / dhildenb