[PATCH 4/7] KVM: arm64: guest debug, add SW break point support

[alex.bennee@linaro.org (Alex Bennée)]

This adds support for SW breakpoints inserted by userspace.

First we need to trap all BKPT exceptions in the hypervisor (ELS). This
in controlled through the MDCR_EL2 register. I've added a new field to
the vcpu structure to hold this value. There should be scope to
rationlise this with the VCPU_DEBUG_FLAGS/KVM_ARM64_DEBUG_DIRTY_SHIFT
manipulation in later patches.

Once the exception arrives we triggers an exit from the main hypervisor
loop to the hypervisor controller. The kvm_debug_exit_arch carries the
address of the exception. If the controller doesn't know of breakpoint
then we have a guest inserted breakpoint and the hypervisor needs to
start again and deliver the exception to guest.

Signed-off-by: Alex Benn?e <alex.bennee@linaro.org>

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 2c6386e..9383359 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -2592,7 +2592,7 @@ when running. Common control bits are:
 The top 16 bits of the control field are architecture specific control
 flags which can include the following:
 
-  - KVM_GUESTDBG_USE_SW_BP:     using software breakpoints [x86]
+  - KVM_GUESTDBG_USE_SW_BP:     using software breakpoints [x86, arm64]
   - KVM_GUESTDBG_USE_HW_BP:     using hardware breakpoints [x86, s390]
   - KVM_GUESTDBG_INJECT_DB:     inject DB type exception [x86]
   - KVM_GUESTDBG_INJECT_BP:     inject BP type exception [x86]
diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
index a0ff410..48d26bb 100644
--- a/arch/arm/kvm/arm.c
+++ b/arch/arm/kvm/arm.c
@@ -303,6 +303,9 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 					struct kvm_guest_debug *dbg)
 {
+	/* Route debug traps to el2? */
+	bool route_el2 = false;
+
 	/* If it's not enabled clear all flags */
 	if (!(dbg->control & KVM_GUESTDBG_ENABLE)) {
 		vcpu->guest_debug = 0;
@@ -320,8 +323,8 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 
 	/* Software Break Points */
 	if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP) {
-		kvm_info("SW BP support requested, not yet implemented\n");
-		return -EINVAL;
+		kvm_info("SW BP support requested\n");
+		route_el2 = true;
 	}
 
 	/* Hardware assisted Break and Watch points */
@@ -330,6 +333,20 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 		return -EINVAL;
 	}
 
+	/* If we are going to handle any debug exceptions we need to
+	 * set MDCE_EL2.TDE to ensure they are routed to the
+	 * hypervisor. If userspace determines the exception was
+	 * actually internal to the guest it needs to handle
+	 * re-injecting the exception.
+	 */
+	if (route_el2) {
+		kvm_info("routing debug exceptions");
+		vcpu->arch.mdcr_el2 = MDCR_EL2_TDE;
+	} else {
+		kvm_info("not routing debug exceptions");
+		vcpu->arch.mdcr_el2 = 0;
+	}
+
 	return 0;
 }
 
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 2012c4b..38b0f07 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -98,6 +98,7 @@ struct kvm_vcpu_arch {
 
 	/* HYP configuration */
 	u64 hcr_el2;
+	u32 mdcr_el2;
 
 	/* Exception Information */
 	struct kvm_vcpu_fault_info fault;
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 9a9fce0..8da1043 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -122,6 +122,7 @@ int main(void)
   DEFINE(VCPU_HPFAR_EL2,	offsetof(struct kvm_vcpu, arch.fault.hpfar_el2));
   DEFINE(VCPU_DEBUG_FLAGS,	offsetof(struct kvm_vcpu, arch.debug_flags));
   DEFINE(VCPU_HCR_EL2,		offsetof(struct kvm_vcpu, arch.hcr_el2));
+  DEFINE(VCPU_MDCR_EL2,	offsetof(struct kvm_vcpu, arch.mdcr_el2));
   DEFINE(VCPU_IRQ_LINES,	offsetof(struct kvm_vcpu, arch.irq_lines));
   DEFINE(VCPU_HOST_CONTEXT,	offsetof(struct kvm_vcpu, arch.host_cpu_context));
   DEFINE(VCPU_TIMER_CNTV_CTL,	offsetof(struct kvm_vcpu, arch.timer_cpu.cntv_ctl));
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 34b8bd0..28dc92b 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -71,6 +71,26 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu, struct kvm_run *run)
 	return 1;
 }
 
+/**
+ * kvm_handle_bkpt - handle a break-point instruction
+ *
+ * @vcpu:	the vcpu pointer
+ *
+ * By definition if we arrive here it's because we are routing
+ * all SW breakpoints to the hyper-visor as some may be a result of
+ * guest debugging. If user-space decides this particular break-point
+ * isn't for the host to handle it can re-feed the exception to the
+ * guest.
+ */
+static int kvm_handle_bkpt(struct kvm_vcpu *vcpu, struct kvm_run *run)
+{
+	run->exit_reason = KVM_EXIT_DEBUG;
+	run->debug.arch.exit_type = KVM_DEBUG_EXIT_SW_BKPT;
+	run->debug.arch.address = *vcpu_pc(vcpu);
+	kvm_info("exiting from %llx\n", run->debug.arch.address);
+	return 0;
+}
+
 static exit_handle_fn arm_exit_handlers[] = {
 	[ESR_EL2_EC_WFI]	= kvm_handle_wfx,
 	[ESR_EL2_EC_CP15_32]	= kvm_handle_cp15_32,
@@ -85,6 +105,8 @@ static exit_handle_fn arm_exit_handlers[] = {
 	[ESR_EL2_EC_SYS64]	= kvm_handle_sys_reg,
 	[ESR_EL2_EC_IABT]	= kvm_handle_guest_abort,
 	[ESR_EL2_EC_DABT]	= kvm_handle_guest_abort,
+	[ESR_EL2_EC_BKPT32]	= kvm_handle_bkpt,
+	[ESR_EL2_EC_BRK64]	= kvm_handle_bkpt,
 };
 
 static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
index b72aa9f..3c733ea 100644
--- a/arch/arm64/kvm/hyp.S
+++ b/arch/arm64/kvm/hyp.S
@@ -772,6 +772,10 @@
 	orr	x2, x2, #(MDCR_EL2_TPM | MDCR_EL2_TPMCR)
 	orr	x2, x2, #(MDCR_EL2_TDRA | MDCR_EL2_TDOSA)
 
+	// Any other bits (currently TDE)
+	ldr	x3, [x0, #VCPU_MDCR_EL2]
+	orr	x2, x2, x3
+
 	// Check for KVM_ARM64_DEBUG_DIRTY, and set debug to trap
 	// if not dirty.
 	ldr	x3, [x0, #VCPU_DEBUG_FLAGS]
-- 
2.1.3