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

From: Laurent Vivier <[email-masked]>
Date: Fri, 8 Apr 2016 08:58:35 +0200

On 08/04/2016 08:23, Thomas Huth wrote:
> 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?

You're right but it adds complexity and it would be only useful to
single-step the single-step mode of the guest.

It's hard to imagine a developer single-stepping the guest kernel while
he is single-stepping a user application in the guest.

It's why I have completely by-passed this case.

Thanks,
Laurent