Subject: Re: [PATCH] kvm-pr: manage single-step mode

From: Thomas Huth <[email-masked]>
Date: Fri, 8 Apr 2016 08:23:01 +0200

On 22.03.2016 15:53, Laurent Vivier wrote:
> Until now, when we connect gdb to the QEMU gdb-server, the
> single-step mode is not managed.
> 
> This patch adds this, only for kvm-pr:
> 
> If KVM_GUESTDBG_SINGLESTEP is set, we enable single-step trace bit in the
> MSR (MSR_SE) just before the __kvmppc_vcpu_run(), and disable it just after.
> In kvmppc_handle_exit_pr, instead of routing the interrupt to
> the guest, we return to host, with KVM_EXIT_DEBUG reason.
> 
> Signed-off-by: Laurent Vivier <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_pr.c | 31 +++++++++++++++++++++++++++++--
>  1 file changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
> index 95bceca..e6896f4 100644
> --- a/arch/powerpc/kvm/book3s_pr.c
> +++ b/arch/powerpc/kvm/book3s_pr.c
> @@ -882,6 +882,24 @@ void kvmppc_set_fscr(struct kvm_vcpu *vcpu, u64 fscr)
>  }
>  #endif
>  
> +static void kvmppc_setup_debug(struct kvm_vcpu *vcpu)
> +{
> +	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> +		u64 msr = kvmppc_get_msr(vcpu);
> +
> +		kvmppc_set_msr(vcpu, msr | MSR_SE);
> +	}
> +}
> +
> +static void kvmppc_clear_debug(struct kvm_vcpu *vcpu)
> +{
> +	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> +		u64 msr = kvmppc_get_msr(vcpu);
> +
> +		kvmppc_set_msr(vcpu, msr & ~MSR_SE);
> +	}
> +}
> +
>  int kvmppc_handle_exit_pr(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  			  unsigned int exit_nr)
>  {
> @@ -1208,8 +1226,13 @@ program_interrupt:
>  #endif
>  	case BOOK3S_INTERRUPT_MACHINE_CHECK:
>  	case BOOK3S_INTERRUPT_TRACE:
> -		kvmppc_book3s_queue_irqprio(vcpu, exit_nr);
> -		r = RESUME_GUEST;
> +		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> +			run->exit_reason = KVM_EXIT_DEBUG;
> +			r = RESUME_HOST;
> +		} else {
> +			kvmppc_book3s_queue_irqprio(vcpu, exit_nr);
> +			r = RESUME_GUEST;
> +		}

Should the new code rather be limited to the BOOK3S_INTERRUPT_TRACE case
only? I mean, this way, you never can deliver a machine check interrupt
to the guest if singlestep debugging is enabled on the host, can you?

 Thomas