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

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Mon, 24 Jul 2023 11:44:10 +0200
Ilya Leoshkevich <iii@linux.ibm.com> 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>

Reviewed-by: Claudio Imbrenda <imbrenda@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,