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

Laurent Vivier
2016-04-08T08:03:09Z

On 08/04/2016 09:44, Thomas Huth wrote:
> On 08.04.2016 08:58, Laurent Vivier wrote:
>>
>>
>> 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 <lvivier@redhat.com>
>>>> ---
>>>> 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.
>
> Hmm, not sure whether you've got me right ;-) I rather meant: What

Yes, I've missed what you mean. :(
Thank you to try again :)

> happens when a machine check is supposed to happen in the guest while
> single stepping is enabled at the host level? IMHO it would be better to
> shape the code like this:
>
> case BOOK3S_INTERRUPT_MACHINE_CHECK:
> kvmppc_book3s_queue_irqprio(vcpu, exit_nr);
> r = RESUME_GUEST;
> break;
> case BOOK3S_INTERRUPT_TRACE:
> 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;
> }
>
> That means, split the two cases, to keep the old behavior for the
> MACHINE_CHECK case. That's also not too much of additional complexity,
> is it?

Yes, you're right.

Thanks,
Laurent