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Date:   Tue, 25 Jul 2023 10:58:56 +0200
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     Ilya Leoshkevich <iii@linux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        Sven Schnelle <svens@linux.ibm.com>, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jens Freimann <jfreimann@redhat.com>
Subject: Re: [PATCH v3 2/6] KVM: s390: interrupt: Fix single-stepping into
 program interrupt handlers
Message-ID: <20230725105856.0ea59d3d@p-imbrenda>
In-Reply-To: <20230724094716.91510-3-iii@linux.ibm.com>
References: <20230724094716.91510-1-iii@linux.ibm.com>
        <20230724094716.91510-3-iii@linux.ibm.com>
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On Mon, 24 Jul 2023 11:44:08 +0200
Ilya Leoshkevich <iii@linux.ibm.com> wrote:

> Currently, after single-stepping an instruction that generates a
> specification exception, GDB ends up on the instruction immediately
> following it.
> 
> The reason is that vcpu_post_run() injects the interrupt and sets
> KVM_GUESTDBG_EXIT_PENDING, causing a KVM_SINGLESTEP exit. The
> interrupt is not delivered, however, therefore userspace sees the
> address of the next instruction.
> 
> Fix by letting the __vcpu_run() loop go into the next iteration,
> where vcpu_pre_run() delivers the interrupt and sets
> KVM_GUESTDBG_EXIT_PENDING.
> 
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  arch/s390/kvm/intercept.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
> index 954d39adf85c..7cdd927541b0 100644
> --- a/arch/s390/kvm/intercept.c
> +++ b/arch/s390/kvm/intercept.c
> @@ -226,7 +226,22 @@ static int handle_itdb(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> -#define per_event(vcpu) (vcpu->arch.sie_block->iprcc & PGM_PER)
> +static bool should_handle_per_event(const struct kvm_vcpu *vcpu)
> +{
> +	if (!guestdbg_enabled(vcpu))
> +		return false;
> +	if (!(vcpu->arch.sie_block->iprcc & PGM_PER))

why not  if (!per_event(vcpu))  ?

maybe you can even merge it with the previous if:

if (!guestdbg_enabled(vcpu) || !per_event(vcpu))
	return false;

this is closer to the old code too

> +		return false;
> +	if (guestdbg_sstep_enabled(vcpu) &&
> +	    vcpu->arch.sie_block->iprcc != PGM_PER) {
> +		/*
> +		 * __vcpu_run() will exit after delivering the concurrently
> +		 * indicated condition.
> +		 */
> +		return false;
> +	}
> +	return true;
> +}
>  
>  static int handle_prog(struct kvm_vcpu *vcpu)
>  {
> @@ -242,7 +257,7 @@ static int handle_prog(struct kvm_vcpu *vcpu)
>  	if (kvm_s390_pv_cpu_is_protected(vcpu))
>  		return -EOPNOTSUPP;
>  
> -	if (guestdbg_enabled(vcpu) && per_event(vcpu)) {
> +	if (should_handle_per_event(vcpu)) {
>  		rc = kvm_s390_handle_per_event(vcpu);
>  		if (rc)
>  			return rc;