RISC-V: KVM: add KVM_CAP_RISCV_MP_STATE_RESET

Add a toggleable VM capability to reset the VCPU from userspace by
setting MP_STATE_INIT_RECEIVED through IOCTL.

Reset through a mp_state to avoid adding a new IOCTL.
Do not reset on a transition from STOPPED to RUNNABLE, because it's
better to avoid side effects that would complicate userspace adoption.
The MP_STATE_INIT_RECEIVED is not a permanent mp_state -- IOCTL resets
the VCPU while preserving the original mp_state -- because we wouldn't
gain much from having a new state it in the rest of KVM, but it's a very
non-standard use of the IOCTL.

Signed-off-by: Radim Krčmář <rkrcmar@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20250515143723.2450630-5-rkrcmar@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>

Documentation/virt/kvm/api.rst

@@ -8496,6 +8496,17 @@ aforementioned registers before the first KVM_RUN. These registers are VM
 scoped, meaning that the same set of values are presented on all vCPUs in a
 given VM.
 
+7.43 KVM_CAP_RISCV_MP_STATE_RESET
+---------------------------------
+
+:Architectures: riscv
+:Type: VM
+:Parameters: None
+:Returns: 0 on success, -EINVAL if arg[0] is not zero
+
+When this capability is enabled, KVM resets the VCPU when setting
+MP_STATE_INIT_RECEIVED through IOCTL.  The original MP_STATE is preserved.
+
 8. Other capabilities.
 ======================
 

arch/riscv/include/asm/kvm_host.h

@@ -119,6 +119,9 @@ struct kvm_arch {
 
 	/* AIA Guest/VM context */
 	struct kvm_aia aia;
+
+	/* KVM_CAP_RISCV_MP_STATE_RESET */
+	bool mp_state_reset;
 };
 
 struct kvm_cpu_trap {

arch/riscv/include/asm/kvm_vcpu_sbi.h

@@ -57,6 +57,7 @@ void kvm_riscv_vcpu_sbi_system_reset(struct kvm_vcpu *vcpu,
 				     u32 type, u64 flags);
 void kvm_riscv_vcpu_sbi_request_reset(struct kvm_vcpu *vcpu,
 				      unsigned long pc, unsigned long a1);
+void kvm_riscv_vcpu_sbi_load_reset_state(struct kvm_vcpu *vcpu);
 int kvm_riscv_vcpu_sbi_return(struct kvm_vcpu *vcpu, struct kvm_run *run);
 int kvm_riscv_vcpu_set_reg_sbi_ext(struct kvm_vcpu *vcpu,
 				   const struct kvm_one_reg *reg);

arch/riscv/kvm/vcpu.c

@@ -51,11 +51,11 @@ const struct kvm_stats_header kvm_vcpu_stats_header = {
 		       sizeof(kvm_vcpu_stats_desc),
 };
 
-static void kvm_riscv_vcpu_context_reset(struct kvm_vcpu *vcpu)
+static void kvm_riscv_vcpu_context_reset(struct kvm_vcpu *vcpu,
+					 bool kvm_sbi_reset)
 {
 	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
 	struct kvm_cpu_context *cntx = &vcpu->arch.guest_context;
-	struct kvm_vcpu_reset_state *reset_state = &vcpu->arch.reset_state;
 	void *vector_datap = cntx->vector.datap;
 
 	memset(cntx, 0, sizeof(*cntx));
@@ -65,13 +65,8 @@ static void kvm_riscv_vcpu_context_reset(struct kvm_vcpu *vcpu)
 	/* Restore datap as it's not a part of the guest context. */
 	cntx->vector.datap = vector_datap;
 
-	/* Load SBI reset values */
-	cntx->a0 = vcpu->vcpu_id;
-
-	spin_lock(&reset_state->lock);
-	cntx->sepc = reset_state->pc;
-	cntx->a1 = reset_state->a1;
-	spin_unlock(&reset_state->lock);
+	if (kvm_sbi_reset)
+		kvm_riscv_vcpu_sbi_load_reset_state(vcpu);
 
 	/* Setup reset state of shadow SSTATUS and HSTATUS CSRs */
 	cntx->sstatus = SR_SPP | SR_SPIE;
@@ -81,7 +76,7 @@ static void kvm_riscv_vcpu_context_reset(struct kvm_vcpu *vcpu)
 	cntx->hstatus |= HSTATUS_SPV;
 }
 
-static void kvm_riscv_reset_vcpu(struct kvm_vcpu *vcpu)
+static void kvm_riscv_reset_vcpu(struct kvm_vcpu *vcpu, bool kvm_sbi_reset)
 {
 	bool loaded;
 
@@ -97,7 +92,7 @@ static void kvm_riscv_reset_vcpu(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.last_exit_cpu = -1;
 
-	kvm_riscv_vcpu_context_reset(vcpu);
+	kvm_riscv_vcpu_context_reset(vcpu, kvm_sbi_reset);
 
 	kvm_riscv_vcpu_fp_reset(vcpu);
 
@@ -174,7 +169,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 	kvm_riscv_vcpu_sbi_init(vcpu);
 
 	/* Reset VCPU */
-	kvm_riscv_reset_vcpu(vcpu);
+	kvm_riscv_reset_vcpu(vcpu, false);
 
 	return 0;
 }
@@ -523,6 +518,12 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
 	case KVM_MP_STATE_STOPPED:
 		__kvm_riscv_vcpu_power_off(vcpu);
 		break;
+	case KVM_MP_STATE_INIT_RECEIVED:
+		if (vcpu->kvm->arch.mp_state_reset)
+			kvm_riscv_reset_vcpu(vcpu, false);
+		else
+			ret = -EINVAL;
+		break;
 	default:
 		ret = -EINVAL;
 	}
@@ -711,7 +712,7 @@ static void kvm_riscv_check_vcpu_requests(struct kvm_vcpu *vcpu)
 		}
 
 		if (kvm_check_request(KVM_REQ_VCPU_RESET, vcpu))
-			kvm_riscv_reset_vcpu(vcpu);
+			kvm_riscv_reset_vcpu(vcpu, true);
 
 		if (kvm_check_request(KVM_REQ_UPDATE_HGATP, vcpu))
 			kvm_riscv_gstage_update_hgatp(vcpu);

arch/riscv/kvm/vcpu_sbi.c

@@ -167,6 +167,23 @@ void kvm_riscv_vcpu_sbi_request_reset(struct kvm_vcpu *vcpu,
 	kvm_make_request(KVM_REQ_VCPU_RESET, vcpu);
 }
 
+void kvm_riscv_vcpu_sbi_load_reset_state(struct kvm_vcpu *vcpu)
+{
+	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
+	struct kvm_cpu_context *cntx = &vcpu->arch.guest_context;
+	struct kvm_vcpu_reset_state *reset_state = &vcpu->arch.reset_state;
+
+	cntx->a0 = vcpu->vcpu_id;
+
+	spin_lock(&vcpu->arch.reset_state.lock);
+	cntx->sepc = reset_state->pc;
+	cntx->a1 = reset_state->a1;
+	spin_unlock(&vcpu->arch.reset_state.lock);
+
+	cntx->sstatus &= ~SR_SIE;
+	csr->vsatp = 0;
+}
+
 int kvm_riscv_vcpu_sbi_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
 {
 	struct kvm_cpu_context *cp = &vcpu->arch.guest_context;

arch/riscv/kvm/vm.c

@@ -209,6 +209,19 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	return r;
 }
 
+int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
+{
+	switch (cap->cap) {
+	case KVM_CAP_RISCV_MP_STATE_RESET:
+		if (cap->flags)
+			return -EINVAL;
+		kvm->arch.mp_state_reset = true;
+		return 0;
+	default:
+		return -EINVAL;
+	}
+}
+
 int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 {
 	return -EINVAL;

include/uapi/linux/kvm.h

@@ -930,6 +930,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
 #define KVM_CAP_X86_GUEST_MODE 238
 #define KVM_CAP_ARM_WRITABLE_IMP_ID_REGS 239
+#define KVM_CAP_RISCV_MP_STATE_RESET 240
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;