RISC-V: KVM: provide UAPI for host SATP mode

KVM userspaces need to be aware of the host SATP to allow them to
advertise it back to the guest OS.

Since this information is used to build the guest FDT we can't wait for
the SATP reg to be readable. We just need to read the SATP mode, thus
we can use the existing 'satp_mode' global that represents the SATP reg
with MODE set and both ASID and PPN cleared. E.g. for a 32 bit host
running with sv32 satp_mode is 0x80000000, for a 64 bit host running
sv57 satp_mode is 0xa000000000000000, and so on.

Add a new userspace virtual config register 'satp_mode' to allow
userspace to read the current SATP mode the host is using with
GET_ONE_REG API before spinning the vcpu.

'satp_mode' can't be changed via KVM, so SET_ONE_REG is allowed as long
as userspace writes the existing 'satp_mode'.
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/include/asm/csr.h

@@ -54,6 +54,7 @@
 #ifndef CONFIG_64BIT
 #define SATP_PPN	_AC(0x003FFFFF, UL)
 #define SATP_MODE_32	_AC(0x80000000, UL)
+#define SATP_MODE_SHIFT	31
 #define SATP_ASID_BITS	9
 #define SATP_ASID_SHIFT	22
 #define SATP_ASID_MASK	_AC(0x1FF, UL)
@@ -62,6 +63,7 @@
 #define SATP_MODE_39	_AC(0x8000000000000000, UL)
 #define SATP_MODE_48	_AC(0x9000000000000000, UL)
 #define SATP_MODE_57	_AC(0xa000000000000000, UL)
+#define SATP_MODE_SHIFT	60
 #define SATP_ASID_BITS	16
 #define SATP_ASID_SHIFT	44
 #define SATP_ASID_MASK	_AC(0xFFFF, UL)

arch/riscv/include/uapi/asm/kvm.h

@@ -55,6 +55,7 @@ struct kvm_riscv_config {
 	unsigned long marchid;
 	unsigned long mimpid;
 	unsigned long zicboz_block_size;
+	unsigned long satp_mode;
 };
 
 /* CORE registers for KVM_GET_ONE_REG and KVM_SET_ONE_REG */

arch/riscv/kvm/vcpu_onereg.c

@@ -152,6 +152,9 @@ static int kvm_riscv_vcpu_get_reg_config(struct kvm_vcpu *vcpu,
 	case KVM_REG_RISCV_CONFIG_REG(mimpid):
 		reg_val = vcpu->arch.mimpid;
 		break;
+	case KVM_REG_RISCV_CONFIG_REG(satp_mode):
+		reg_val = satp_mode >> SATP_MODE_SHIFT;
+		break;
 	default:
 		return -EINVAL;
 	}
@@ -234,6 +237,10 @@ static int kvm_riscv_vcpu_set_reg_config(struct kvm_vcpu *vcpu,
 		else
 			return -EBUSY;
 		break;
+	case KVM_REG_RISCV_CONFIG_REG(satp_mode):
+		if (reg_val != (satp_mode >> SATP_MODE_SHIFT))
+			return -EINVAL;
+		break;
 	default:
 		return -EINVAL;
 	}