KVM: arm64: Reload vCPU for accesses to OSLAR_EL1

KVM takes ownership of the debug regs if the guest enables the OS lock,
as it needs to use MDSCR_EL1 to mask debug exceptions. Just reload the
vCPU if the guest toggles the OS lock, relying on kvm_vcpu_load_debug()
to update the debug owner and get the right trap configuration in place.

Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20241219224116.3941496-13-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/include/asm/kvm_host.h

@@ -1346,6 +1346,7 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
 void kvm_arm_clear_debug(struct kvm_vcpu *vcpu);
 void kvm_vcpu_load_debug(struct kvm_vcpu *vcpu);
 void kvm_debug_set_guest_ownership(struct kvm_vcpu *vcpu);
+void kvm_debug_handle_oslar(struct kvm_vcpu *vcpu, u64 val);
 
 #define kvm_vcpu_os_lock_enabled(vcpu)		\
 	(!!(__vcpu_sys_reg(vcpu, OSLSR_EL1) & OSLSR_EL1_OSLK))

arch/arm64/kvm/debug.c

@@ -288,3 +288,16 @@ void kvm_debug_set_guest_ownership(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.debug_owner = VCPU_DEBUG_GUEST_OWNED;
 }
+
+void kvm_debug_handle_oslar(struct kvm_vcpu *vcpu, u64 val)
+{
+	if (val & OSLAR_EL1_OSLK)
+		__vcpu_sys_reg(vcpu, OSLSR_EL1) |= OSLSR_EL1_OSLK;
+	else
+		__vcpu_sys_reg(vcpu, OSLSR_EL1) &= ~OSLSR_EL1_OSLK;
+
+	preempt_disable();
+	kvm_arch_vcpu_put(vcpu);
+	kvm_arch_vcpu_load(vcpu, smp_processor_id());
+	preempt_enable();
+}

arch/arm64/kvm/sys_regs.c

@@ -570,17 +570,10 @@ static bool trap_oslar_el1(struct kvm_vcpu *vcpu,
 			   struct sys_reg_params *p,
 			   const struct sys_reg_desc *r)
 {
-	u64 oslsr;
-
 	if (!p->is_write)
 		return read_from_write_only(vcpu, p, r);
 
-	/* Forward the OSLK bit to OSLSR */
-	oslsr = __vcpu_sys_reg(vcpu, OSLSR_EL1) & ~OSLSR_EL1_OSLK;
-	if (p->regval & OSLAR_EL1_OSLK)
-		oslsr |= OSLSR_EL1_OSLK;
-
-	__vcpu_sys_reg(vcpu, OSLSR_EL1) = oslsr;
+	kvm_debug_handle_oslar(vcpu, p->regval);
 	return true;
 }