arm64: KVM: Consistently advance singlestep when emulating instructions

When we emulate a guest instruction, we don't advance the hardware
singlestep state machine, and thus the guest will receive a software
step exception after a next instruction which is not emulated by the
host.

We bodge around this in an ad-hoc fashion. Sometimes we explicitly check
whether userspace requested a single step, and fake a debug exception
from within the kernel. Other times, we advance the HW singlestep state
rely on the HW to generate the exception for us. Thus, the observed step
behaviour differs for host and guest.

Let's make this simpler and consistent by always advancing the HW
singlestep state machine when we skip an instruction. Thus we can rely
on the hardware to generate the singlestep exception for us, and never
need to explicitly check for an active-pending step, nor do we need to
fake a debug exception from the guest.

Cc: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

1 files changed, 1 insertions, 13 deletions

diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 35a81bebd02b..b0643f9c4873 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -229,13 +229,6 @@ static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
 		handled = exit_handler(vcpu, run);
 	}
 
-	/*
-	 * kvm_arm_handle_step_debug() sets the exit_reason on the kvm_run
-	 * structure if we need to return to userspace.
-	 */
-	if (handled > 0 && kvm_arm_handle_step_debug(vcpu, run))
-		handled = 0;
-
 	return handled;
 }
 
@@ -269,12 +262,7 @@ int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
 	case ARM_EXCEPTION_IRQ:
 		return 1;
 	case ARM_EXCEPTION_EL1_SERROR:
-		/* We may still need to return for single-step */
-		if (!(*vcpu_cpsr(vcpu) & DBG_SPSR_SS)
-			&& kvm_arm_handle_step_debug(vcpu, run))
-			return 0;
-		else
-			return 1;
+		return 1;
 	case ARM_EXCEPTION_TRAP:
 		return handle_trap_exceptions(vcpu, run);
 	case ARM_EXCEPTION_HYP_GONE: