arm/arm64: KVM: Allow a VCPU to fully reset itself

The current kvm_psci_vcpu_on implementation will directly try to
manipulate the state of the VCPU to reset it.  However, since this is
not done on the thread that runs the VCPU, we can end up in a strangely
corrupted state when the source and target VCPUs are running at the same
time.

Fix this by factoring out all reset logic from the PSCI implementation
and forwarding the required information along with a request to the
target VCPU.
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>

arch/arm/include/asm/kvm_host.h

@@ -48,6 +48,7 @@
 #define KVM_REQ_SLEEP \
 	KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_IRQ_PENDING	KVM_ARCH_REQ(1)
+#define KVM_REQ_VCPU_RESET	KVM_ARCH_REQ(2)
 
 DECLARE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
 
@@ -147,6 +148,13 @@ struct kvm_cpu_context {
 
 typedef struct kvm_cpu_context kvm_cpu_context_t;
 
+struct vcpu_reset_state {
+	unsigned long	pc;
+	unsigned long	r0;
+	bool		be;
+	bool		reset;
+};
+
 struct kvm_vcpu_arch {
 	struct kvm_cpu_context ctxt;
 
@@ -186,6 +194,8 @@ struct kvm_vcpu_arch {
 	/* Cache some mmu pages needed inside spinlock regions */
 	struct kvm_mmu_memory_cache mmu_page_cache;
 
+	struct vcpu_reset_state reset_state;
+
 	/* Detect first run of a vcpu */
 	bool has_run_once;
 };

arch/arm/kvm/reset.c

@@ -26,6 +26,7 @@
 #include <asm/cputype.h>
 #include <asm/kvm_arm.h>
 #include <asm/kvm_coproc.h>
+#include <asm/kvm_emulate.h>
 
 #include <kvm/arm_arch_timer.h>
 
@@ -69,6 +70,29 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 	/* Reset CP15 registers */
 	kvm_reset_coprocs(vcpu);
 
+	/*
+	 * Additional reset state handling that PSCI may have imposed on us.
+	 * Must be done after all the sys_reg reset.
+	 */
+	if (READ_ONCE(vcpu->arch.reset_state.reset)) {
+		unsigned long target_pc = vcpu->arch.reset_state.pc;
+
+		/* Gracefully handle Thumb2 entry point */
+		if (target_pc & 1) {
+			target_pc &= ~1UL;
+			vcpu_set_thumb(vcpu);
+		}
+
+		/* Propagate caller endianness */
+		if (vcpu->arch.reset_state.be)
+			kvm_vcpu_set_be(vcpu);
+
+		*vcpu_pc(vcpu) = target_pc;
+		vcpu_set_reg(vcpu, 0, vcpu->arch.reset_state.r0);
+
+		vcpu->arch.reset_state.reset = false;
+	}
+
 	/* Reset arch_timer context */
 	return kvm_timer_vcpu_reset(vcpu);
 }

arch/arm64/include/asm/kvm_host.h

@@ -48,6 +48,7 @@
 #define KVM_REQ_SLEEP \
 	KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_IRQ_PENDING	KVM_ARCH_REQ(1)
+#define KVM_REQ_VCPU_RESET	KVM_ARCH_REQ(2)
 
 DECLARE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
 
@@ -208,6 +209,13 @@ struct kvm_cpu_context {
 
 typedef struct kvm_cpu_context kvm_cpu_context_t;
 
+struct vcpu_reset_state {
+	unsigned long	pc;
+	unsigned long	r0;
+	bool		be;
+	bool		reset;
+};
+
 struct kvm_vcpu_arch {
 	struct kvm_cpu_context ctxt;
 
@@ -297,6 +305,9 @@ struct kvm_vcpu_arch {
 	/* Virtual SError ESR to restore when HCR_EL2.VSE is set */
 	u64 vsesr_el2;
 
+	/* Additional reset state */
+	struct vcpu_reset_state	reset_state;
+
 	/* True when deferrable sysregs are loaded on the physical CPU,
 	 * see kvm_vcpu_load_sysregs and kvm_vcpu_put_sysregs. */
 	bool sysregs_loaded_on_cpu;

arch/arm64/kvm/reset.c

@@ -32,6 +32,7 @@
 #include <asm/kvm_arm.h>
 #include <asm/kvm_asm.h>
 #include <asm/kvm_coproc.h>
+#include <asm/kvm_emulate.h>
 #include <asm/kvm_mmu.h>
 
 /* Maximum phys_shift supported for any VM on this host */
@@ -146,6 +147,29 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 	/* Reset system registers */
 	kvm_reset_sys_regs(vcpu);
 
+	/*
+	 * Additional reset state handling that PSCI may have imposed on us.
+	 * Must be done after all the sys_reg reset.
+	 */
+	if (vcpu->arch.reset_state.reset) {
+		unsigned long target_pc = vcpu->arch.reset_state.pc;
+
+		/* Gracefully handle Thumb2 entry point */
+		if (vcpu_mode_is_32bit(vcpu) && (target_pc & 1)) {
+			target_pc &= ~1UL;
+			vcpu_set_thumb(vcpu);
+		}
+
+		/* Propagate caller endianness */
+		if (vcpu->arch.reset_state.be)
+			kvm_vcpu_set_be(vcpu);
+
+		*vcpu_pc(vcpu) = target_pc;
+		vcpu_set_reg(vcpu, 0, vcpu->arch.reset_state.r0);
+
+		vcpu->arch.reset_state.reset = false;
+	}
+
 	/* Reset PMU */
 	kvm_pmu_vcpu_reset(vcpu);
 

virt/kvm/arm/arm.c

@@ -626,6 +626,13 @@ static void vcpu_req_sleep(struct kvm_vcpu *vcpu)
 		/* Awaken to handle a signal, request we sleep again later. */
 		kvm_make_request(KVM_REQ_SLEEP, vcpu);
 	}
+
+	/*
+	 * Make sure we will observe a potential reset request if we've
+	 * observed a change to the power state. Pairs with the smp_wmb() in
+	 * kvm_psci_vcpu_on().
+	 */
+	smp_rmb();
 }
 
 static int kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
@@ -639,6 +646,9 @@ static void check_vcpu_requests(struct kvm_vcpu *vcpu)
 		if (kvm_check_request(KVM_REQ_SLEEP, vcpu))
 			vcpu_req_sleep(vcpu);
 
+		if (kvm_check_request(KVM_REQ_VCPU_RESET, vcpu))
+			kvm_reset_vcpu(vcpu);
+
 		/*
 		 * Clear IRQ_PENDING requests that were made to guarantee
 		 * that a VCPU sees new virtual interrupts.

virt/kvm/arm/psci.c

@@ -104,12 +104,10 @@ static void kvm_psci_vcpu_off(struct kvm_vcpu *vcpu)
 
 static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu)
 {
+	struct vcpu_reset_state *reset_state;
 	struct kvm *kvm = source_vcpu->kvm;
 	struct kvm_vcpu *vcpu = NULL;
-	struct swait_queue_head *wq;
 	unsigned long cpu_id;
-	unsigned long context_id;
-	phys_addr_t target_pc;
 
 	cpu_id = smccc_get_arg1(source_vcpu) & MPIDR_HWID_BITMASK;
 	if (vcpu_mode_is_32bit(source_vcpu))
@@ -130,32 +128,30 @@ static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu)
 			return PSCI_RET_INVALID_PARAMS;
 	}
 
-	target_pc = smccc_get_arg2(source_vcpu);
-	context_id = smccc_get_arg3(source_vcpu);
+	reset_state = &vcpu->arch.reset_state;
 
-	kvm_reset_vcpu(vcpu);
-
-	/* Gracefully handle Thumb2 entry point */
-	if (vcpu_mode_is_32bit(vcpu) && (target_pc & 1)) {
-		target_pc &= ~((phys_addr_t) 1);
-		vcpu_set_thumb(vcpu);
-	}
+	reset_state->pc = smccc_get_arg2(source_vcpu);
 
 	/* Propagate caller endianness */
-	if (kvm_vcpu_is_be(source_vcpu))
-		kvm_vcpu_set_be(vcpu);
+	reset_state->be = kvm_vcpu_is_be(source_vcpu);
 
-	*vcpu_pc(vcpu) = target_pc;
 	/*
 	 * NOTE: We always update r0 (or x0) because for PSCI v0.1
 	 * the general puspose registers are undefined upon CPU_ON.
 	 */
-	smccc_set_retval(vcpu, context_id, 0, 0, 0);
-	vcpu->arch.power_off = false;
-	smp_mb();		/* Make sure the above is visible */
+	reset_state->r0 = smccc_get_arg3(source_vcpu);
+
+	WRITE_ONCE(reset_state->reset, true);
+	kvm_make_request(KVM_REQ_VCPU_RESET, vcpu);
 
-	wq = kvm_arch_vcpu_wq(vcpu);
-	swake_up_one(wq);
+	/*
+	 * Make sure the reset request is observed if the change to
+	 * power_state is observed.
+	 */
+	smp_wmb();
+
+	vcpu->arch.power_off = false;
+	kvm_vcpu_wake_up(vcpu);
 
 	return PSCI_RET_SUCCESS;
 }