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

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 <redacted>
---
 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;
+		}
 		break;
 	default:
 	{

@@ -1479,6 +1502,8 @@ static int kvmppc_vcpu_run_pr(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
 		goto out;
 	}
 
+	kvmppc_setup_debug(vcpu);
+
 	/*
 	 * Interrupts could be timers for the guest which we have to inject
 	 * again, so let's postpone them until we're in the guest and if we

@@ -1501,6 +1526,8 @@ static int kvmppc_vcpu_run_pr(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
 
 	ret = __kvmppc_vcpu_run(kvm_run, vcpu);
 
+	kvmppc_clear_debug(vcpu);
+
 	/* No need for kvm_guest_exit. It's done in handle_exit.
 	   We also get here with interrupts enabled. */
 

-- 
2.5.0

Ping?

On 22/03/2016 15:53, Laurent Vivier wrote:

On 22.03.2016 15:53, Laurent Vivier wrote:

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

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

On 22.03.2016 15:53, Laurent Vivier wrote:

quoted

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 <redacted>
---
 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

On 08.04.2016 08:58, Laurent Vivier wrote:

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

quoted

On 22.03.2016 15:53, Laurent Vivier wrote:

quoted

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 <redacted>
---
 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
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?

 Thomas

On 08/04/2016 09:44, Thomas Huth wrote:

On 08.04.2016 08:58, Laurent Vivier wrote:

quoted

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

quoted

On 22.03.2016 15:53, Laurent Vivier wrote:

quoted

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 <redacted>
---
 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