[PATCH v1 2/2] kvm: arm64: handle single-step of userspace mmio instructions

[Alex Bennée]

The system state of KVM when using userspace emulation is not complete
until we return into KVM_RUN. To handle mmio related updates we wait
until they have been committed and then schedule our KVM_EXIT_DEBUG.

I've introduced a new function kvm_arm_maybe_return_debug() to wrap up
the differences between arm/arm64 which is currently null for arm.

Signed-off-by: Alex Bennée <alex.bennee at linaro.org>
---
 arch/arm/include/asm/kvm_host.h   |  2 ++
 arch/arm64/include/asm/kvm_host.h |  1 +
 arch/arm64/kvm/debug.c            | 21 +++++++++++++++++++++
 arch/arm64/kvm/handle_exit.c      |  9 +++------
 virt/kvm/arm/arm.c                |  2 +-
 virt/kvm/arm/mmio.c               |  3 ++-
 6 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
index 4a879f6ff13b..aec943f6d123 100644
--- a/arch/arm/include/asm/kvm_host.h
+++ b/arch/arm/include/asm/kvm_host.h
@@ -285,6 +285,8 @@ static inline void kvm_arm_init_debug(void) {}
 static inline void kvm_arm_setup_debug(struct kvm_vcpu *vcpu) {}
 static inline void kvm_arm_clear_debug(struct kvm_vcpu *vcpu) {}
 static inline void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu) {}
+static inline int  kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu,
+						struct kvm_run *run) {}
 
 int kvm_arm_vcpu_arch_set_attr(struct kvm_vcpu *vcpu,
 			       struct kvm_device_attr *attr);
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index e923b58606e2..fa67d21662f6 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -369,6 +369,7 @@ void kvm_arm_init_debug(void);
 void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
 void kvm_arm_clear_debug(struct kvm_vcpu *vcpu);
 void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu);
+int  kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu, struct kvm_run *run);
 int kvm_arm_vcpu_arch_set_attr(struct kvm_vcpu *vcpu,
 			       struct kvm_device_attr *attr);
 int kvm_arm_vcpu_arch_get_attr(struct kvm_vcpu *vcpu,
diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index dbadfaf850a7..a10a18c55c87 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c
@@ -221,3 +221,24 @@ void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
 		}
 	}
 }
+
+
+/*
+ * When KVM has successfully emulated the instruction we might want to
+ * return we a KVM_EXIT_DEBUG. We can only do this once the emulation
+ * is complete though so for userspace emulations we have to wait
+ * until we have re-entered KVM.
+ *
+ * Return > 0 to return to guest, 0 (and set exit_reason) on proper
+ * exit to userspace.
+ */
+
+int kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
+{
+	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+		run->exit_reason = KVM_EXIT_DEBUG;
+		run->debug.arch.hsr = ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT;
+		return 0;
+	}
+	return 1;
+}
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index c918d291cb58..7b04f59217bf 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -202,13 +202,10 @@ static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
 		handled = exit_handler(vcpu, run);
 	}
 
-	if (handled && (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)) {
-		handled = 0;
-		run->exit_reason = KVM_EXIT_DEBUG;
-		run->debug.arch.hsr = ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT;
-	}
+	if (handled)
+		return kvm_arm_maybe_return_debug(vcpu, run);
 
-	return handled;
+	return 0;
 }
 
 /*
diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
index b9f68e4add71..3d28fe2daa26 100644
--- a/virt/kvm/arm/arm.c
+++ b/virt/kvm/arm/arm.c
@@ -623,7 +623,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 
 	if (run->exit_reason == KVM_EXIT_MMIO) {
 		ret = kvm_handle_mmio_return(vcpu, vcpu->run);
-		if (ret)
+		if (ret < 1)
 			return ret;
 	}
 
diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
index b6e715fd3c90..e43e3bd6222f 100644
--- a/virt/kvm/arm/mmio.c
+++ b/virt/kvm/arm/mmio.c
@@ -117,7 +117,8 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
 		vcpu_set_reg(vcpu, vcpu->arch.mmio_decode.rt, data);
 	}
 
-	return 0;
+	/* If debugging in effect we may need to return now */
+	return kvm_arm_maybe_return_debug(vcpu, run);
 }
 
 static int decode_hsr(struct kvm_vcpu *vcpu, bool *is_write, int *len)
-- 
2.14.1