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

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Message-ID: <170e3285-2e01-840e-21bc-39dbad256542@redhat.com>
Date:   Mon, 24 Jul 2023 13:31:14 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ilya Leoshkevich <iii@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     Sven Schnelle <svens@...ux.ibm.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Freimann <jfreimann@...hat.com>
Subject: Re: [PATCH v3 4/6] KVM: s390: interrupt: Fix single-stepping
 userspace-emulated instructions

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@...ux.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@...hat.com>

-- 
Cheers,

David / dhildenb