linux-kernel - Re: [PATCH v2 1/6] KVM: s390: interrupt: Fix single-stepping into interrupt handlers

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Date:   Mon, 24 Jul 2023 10:22:52 +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 v2 1/6] KVM: s390: interrupt: Fix single-stepping into
 interrupt handlers

On 21.07.23 13:57, Ilya Leoshkevich wrote:
> After single-stepping an instruction that generates an interrupt, GDB
> ends up on the second instruction of the respective interrupt handler.
> 
> The reason is that vcpu_pre_run() manually delivers the interrupt, and
> then __vcpu_run() runs the first handler instruction using the
> CPUSTAT_P flag. This causes a KVM_SINGLESTEP exit on the second handler
> instruction.
> 
> Fix by delaying the KVM_SINGLESTEP exit until after the manual
> interrupt delivery.
> 
> Signed-off-by: Ilya Leoshkevich <iii@...ux.ibm.com>
> ---
>   arch/s390/kvm/interrupt.c | 10 ++++++++++
>   arch/s390/kvm/kvm-s390.c  |  4 ++--
>   2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
> index 9bd0a873f3b1..2cebe4227b8e 100644
> --- a/arch/s390/kvm/interrupt.c
> +++ b/arch/s390/kvm/interrupt.c
> @@ -1392,6 +1392,7 @@ int __must_check kvm_s390_deliver_pending_interrupts(struct kvm_vcpu *vcpu)
>   {
>   	struct kvm_s390_local_interrupt *li = &vcpu->arch.local_int;
>   	int rc = 0;
> +	bool delivered = false;
>   	unsigned long irq_type;
>   	unsigned long irqs;
>   
> @@ -1465,6 +1466,15 @@ int __must_check kvm_s390_deliver_pending_interrupts(struct kvm_vcpu *vcpu)
>   			WARN_ONCE(1, "Unknown pending irq type %ld", irq_type);
>   			clear_bit(irq_type, &li->pending_irqs);
>   		}
> +		delivered |= !rc;
> +	}
> +


Can we add a comment like

/*
  * We delivered at least one interrupt and modified the PC. Force a
  * singlestep event now.
  */

> +	if (delivered && guestdbg_sstep_enabled(vcpu)) {
> +		struct kvm_debug_exit_arch *debug_exit = &vcpu->run->debug.arch;
> +
> +		debug_exit->addr = vcpu->arch.sie_block->gpsw.addr;
> +		debug_exit->type = KVM_SINGLESTEP;
> +		vcpu->guest_debug |= KVM_GUESTDBG_EXIT_PENDING;
>   	}

I do wonder if we, instead, want to do this whenever we modify the PSW.

That way we could catch any PC changes and only have to add checks for 
guestdbg_exit_pending().


But this is simpler and should work as well.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb