KVM: arm64: Evaluate debug owner at vcpu_load()

In preparation for tossing the debug_ptr mess, introduce an enumeration
to track the ownership of the debug registers while in the guest. Update
the owner at vcpu_load() based on whether the host needs to steal the
guest's debug context or if breakpoints/watchpoints are actively in use.

Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20241219224116.3941496-7-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: oupton

arch/arm64/include/asm/kvm_host.h

@@ -756,6 +756,12 @@ struct kvm_vcpu_arch {
 	struct kvm_guest_debug_arch vcpu_debug_state;
 	struct kvm_guest_debug_arch external_debug_state;
 
+	enum {
+		VCPU_DEBUG_FREE,
+		VCPU_DEBUG_HOST_OWNED,
+		VCPU_DEBUG_GUEST_OWNED,
+	} debug_owner;
+
 	/* VGIC state */
 	struct vgic_cpu vgic_cpu;
 	struct arch_timer_cpu timer_cpu;
@@ -1345,10 +1351,15 @@ 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);
+void kvm_vcpu_load_debug(struct kvm_vcpu *vcpu);
+void kvm_debug_set_guest_ownership(struct kvm_vcpu *vcpu);
 
 #define kvm_vcpu_os_lock_enabled(vcpu)		\
 	(!!(__vcpu_sys_reg(vcpu, OSLSR_EL1) & OSLSR_EL1_OSLK))
 
+#define kvm_host_owns_debug_regs(vcpu)		\
+	((vcpu)->arch.debug_owner == VCPU_DEBUG_HOST_OWNED)
+
 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,

arch/arm64/kvm/arm.c

@@ -598,6 +598,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
 	kvm_vgic_load(vcpu);
 	kvm_timer_vcpu_load(vcpu);
+	kvm_vcpu_load_debug(vcpu);
 	if (has_vhe())
 		kvm_vcpu_load_vhe(vcpu);
 	kvm_arch_vcpu_load_fp(vcpu);

arch/arm64/kvm/debug.c

@@ -317,3 +317,49 @@ void kvm_init_host_debug_data(void)
 	    !(read_sysreg_s(SYS_TRBIDR_EL1) & TRBIDR_EL1_P))
 		host_data_set_flag(HAS_TRBE);
 }
+
+void kvm_vcpu_load_debug(struct kvm_vcpu *vcpu)
+{
+	u64 mdscr;
+
+	/* Must be called before kvm_vcpu_load_vhe() */
+	KVM_BUG_ON(vcpu_get_flag(vcpu, SYSREGS_ON_CPU), vcpu->kvm);
+
+	/*
+	 * Determine which of the possible debug states we're in:
+	 *
+	 *  - VCPU_DEBUG_HOST_OWNED: KVM has taken ownership of the guest's
+	 *    breakpoint/watchpoint registers, or needs to use MDSCR_EL1 to do
+	 *    software step or emulate the effects of the OS Lock being enabled.
+	 *
+	 *  - VCPU_DEBUG_GUEST_OWNED: The guest has debug exceptions enabled, and
+	 *    the breakpoint/watchpoint registers need to be loaded eagerly.
+	 *
+	 *  - VCPU_DEBUG_FREE: Neither of the above apply, no breakpoint/watchpoint
+	 *    context needs to be loaded on the CPU.
+	 */
+	if (vcpu->guest_debug || kvm_vcpu_os_lock_enabled(vcpu)) {
+		vcpu->arch.debug_owner = VCPU_DEBUG_HOST_OWNED;
+	} else {
+		mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
+
+		if (mdscr & (MDSCR_EL1_KDE | MDSCR_EL1_MDE))
+			vcpu->arch.debug_owner = VCPU_DEBUG_GUEST_OWNED;
+		else
+			vcpu->arch.debug_owner = VCPU_DEBUG_FREE;
+	}
+}
+
+/*
+ * Updates ownership of the debug registers after a trapped guest access to a
+ * breakpoint/watchpoint register. Host ownership of the debug registers is of
+ * strictly higher priority, and it is the responsibility of the VMM to emulate
+ * guest debug exceptions in this configuration.
+ */
+void kvm_debug_set_guest_ownership(struct kvm_vcpu *vcpu)
+{
+	if (kvm_host_owns_debug_regs(vcpu))
+		return;
+
+	vcpu->arch.debug_owner = VCPU_DEBUG_GUEST_OWNED;
+}

arch/arm64/kvm/sys_regs.c

@@ -656,6 +656,7 @@ static bool trap_debug_regs(struct kvm_vcpu *vcpu,
 	if (p->is_write)
 		vcpu_set_flag(vcpu, DEBUG_DIRTY);
 
+	kvm_debug_set_guest_ownership(vcpu);
 	trace_trap_reg(__func__, r->reg, p->is_write, p->regval);
 
 	return true;
@@ -684,6 +685,7 @@ static void reg_to_dbg(struct kvm_vcpu *vcpu,
 	val |= (p->regval & (mask >> shift)) << shift;
 	*dbg_reg = val;
 
+	kvm_debug_set_guest_ownership(vcpu);
 	vcpu_set_flag(vcpu, DEBUG_DIRTY);
 }