KVM: arm64: Initialize VCPU mdcr_el2 before loading it

When a VCPU is created, the kvm_vcpu struct is initialized to zero in
kvm_vm_ioctl_create_vcpu(). On VHE systems, the first time
vcpu.arch.mdcr_el2 is loaded on hardware is in vcpu_load(), before it is
set to a sensible value in kvm_arm_setup_debug() later in the run loop. The
result is that KVM executes for a short time with MDCR_EL2 set to zero.

This has several unintended consequences:

* Setting MDCR_EL2.HPMN to 0 is constrained unpredictable according to ARM
  DDI 0487G.a, page D13-3820. The behavior specified by the architecture
  in this case is for the PE to behave as if MDCR_EL2.HPMN is set to a
  value less than or equal to PMCR_EL0.N, which means that an unknown
  number of counters are now disabled by MDCR_EL2.HPME, which is zero.

* The host configuration for the other debug features controlled by
  MDCR_EL2 is temporarily lost. This has been harmless so far, as Linux
  doesn't use the other fields, but that might change in the future.

Let's avoid both issues by initializing the VCPU's mdcr_el2 field in
kvm_vcpu_vcpu_first_run_init(), thus making sure that the MDCR_EL2 register
has a consistent value after each vcpu_load().

Fixes: d5a21bcc2995 ("KVM: arm64: Move common VHE/non-VHE trap config in separate functions")
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210407144857.199746-3-alexandru.elisei@arm.com

3 files changed, 63 insertions, 28 deletions

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 3d10e6527f7d..858c2fcfc043 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -713,6 +713,7 @@ static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
 static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
 
 void kvm_arm_init_debug(void);
+void kvm_arm_vcpu_init_debug(struct kvm_vcpu *vcpu);
 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);
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 7f06ba76698d..455274c704b8 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -580,6 +580,8 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.has_run_once = true;
 
+	kvm_arm_vcpu_init_debug(vcpu);
+
 	if (likely(irqchip_in_kernel(kvm))) {
 		/*
 		 * Map the VGIC hardware resources before running a vcpu the
diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index dbc890511631..2484b2cca74b 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c
@@ -69,6 +69,64 @@ void kvm_arm_init_debug(void)
 }
 
 /**
+ * kvm_arm_setup_mdcr_el2 - configure vcpu mdcr_el2 value
+ *
+ * @vcpu:	the vcpu pointer
+ *
+ * This ensures we will trap access to:
+ *  - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
+ *  - Debug ROM Address (MDCR_EL2_TDRA)
+ *  - OS related registers (MDCR_EL2_TDOSA)
+ *  - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
+ *  - Self-hosted Trace Filter controls (MDCR_EL2_TTRF)
+ */
+static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * This also clears MDCR_EL2_E2PB_MASK to disable guest access
+	 * to the profiling buffer.
+	 */
+	vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) & MDCR_EL2_HPMN_MASK;
+	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
+				MDCR_EL2_TPMS |
+				MDCR_EL2_TTRF |
+				MDCR_EL2_TPMCR |
+				MDCR_EL2_TDRA |
+				MDCR_EL2_TDOSA);
+
+	/* Is the VM being debugged by userspace? */
+	if (vcpu->guest_debug)
+		/* Route all software debug exceptions to EL2 */
+		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
+
+	/*
+	 * Trap debug register access when one of the following is true:
+	 *  - Userspace is using the hardware to debug the guest
+	 *  (KVM_GUESTDBG_USE_HW is set).
+	 *  - The guest is not using debug (KVM_ARM64_DEBUG_DIRTY is clear).
+	 */
+	if ((vcpu->guest_debug & KVM_GUESTDBG_USE_HW) ||
+	    !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY))
+		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
+
+	trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
+}
+
+/**
+ * kvm_arm_vcpu_init_debug - setup vcpu debug traps
+ *
+ * @vcpu:	the vcpu pointer
+ *
+ * Set vcpu initial mdcr_el2 value.
+ */
+void kvm_arm_vcpu_init_debug(struct kvm_vcpu *vcpu)
+{
+	preempt_disable();
+	kvm_arm_setup_mdcr_el2(vcpu);
+	preempt_enable();
+}
+
+/**
  * kvm_arm_reset_debug_ptr - reset the debug ptr to point to the vcpu state
  */
 
@@ -83,13 +141,7 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
  * @vcpu:	the vcpu pointer
  *
  * This is called before each entry into the hypervisor to setup any
- * debug related registers. Currently this just ensures we will trap
- * access to:
- *  - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
- *  - Debug ROM Address (MDCR_EL2_TDRA)
- *  - OS related registers (MDCR_EL2_TDOSA)
- *  - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
- *  - Self-hosted Trace Filter controls (MDCR_EL2_TTRF)
+ * debug related registers.
  *
  * Additionally, KVM only traps guest accesses to the debug registers if
  * the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
@@ -101,28 +153,14 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
 
 void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
 {
-	bool trap_debug = !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY);
 	unsigned long mdscr, orig_mdcr_el2 = vcpu->arch.mdcr_el2;
 
 	trace_kvm_arm_setup_debug(vcpu, vcpu->guest_debug);
 
-	/*
-	 * This also clears MDCR_EL2_E2PB_MASK to disable guest access
-	 * to the profiling buffer.
-	 */
-	vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) & MDCR_EL2_HPMN_MASK;
-	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
-				MDCR_EL2_TPMS |
-				MDCR_EL2_TTRF |
-				MDCR_EL2_TPMCR |
-				MDCR_EL2_TDRA |
-				MDCR_EL2_TDOSA);
+	kvm_arm_setup_mdcr_el2(vcpu);
 
 	/* Is Guest debugging in effect? */
 	if (vcpu->guest_debug) {
-		/* Route all software debug exceptions to EL2 */
-		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
-
 		/* Save guest debug state */
 		save_guest_debug_regs(vcpu);
 
@@ -176,7 +214,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
 
 			vcpu->arch.debug_ptr = &vcpu->arch.external_debug_state;
 			vcpu->arch.flags |= KVM_ARM64_DEBUG_DIRTY;
-			trap_debug = true;
 
 			trace_kvm_arm_set_regset("BKPTS", get_num_brps(),
 						&vcpu->arch.debug_ptr->dbg_bcr[0],
@@ -191,10 +228,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
 	BUG_ON(!vcpu->guest_debug &&
 		vcpu->arch.debug_ptr != &vcpu->arch.vcpu_debug_state);
 
-	/* Trap debug register access */
-	if (trap_debug)
-		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
-
 	/* If KDE or MDE are set, perform a full save/restore cycle. */
 	if (vcpu_read_sys_reg(vcpu, MDSCR_EL1) & (DBG_MDSCR_KDE | DBG_MDSCR_MDE))
 		vcpu->arch.flags |= KVM_ARM64_DEBUG_DIRTY;
@@ -203,7 +236,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
 	if (has_vhe() && orig_mdcr_el2 != vcpu->arch.mdcr_el2)
 		write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2);
 
-	trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
 	trace_kvm_arm_set_dreg32("MDSCR_EL1", vcpu_read_sys_reg(vcpu, MDSCR_EL1));
 }