RISC-V: KVM: avoid EBUSY when writing the same machine ID val

Right now we do not allow any write in mvendorid/marchid/mimpid if the
vcpu already started, preventing these regs to be changed.

However, if userspace doesn't change them, an alternative is to consider
the reg write a no-op and avoid erroring out altogether. Userpace can
then be oblivious about KVM internals if no changes were intended in the
first place.

Allow the same form of 'lazy writing' that registers such as
zicbom/zicboz_block_size supports: avoid erroring out if userspace makes
no changes in mvendorid/marchid/mimpid during reg write.

Signed-off-by: Daniel Henrique Barboza <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
Signed-off-by: Anup Patel <[email protected]>

Author: danielhb

arch/riscv/kvm/vcpu_onereg.c

@@ -235,18 +235,24 @@ static int kvm_riscv_vcpu_set_reg_config(struct kvm_vcpu *vcpu,
 			return -EINVAL;
 		break;
 	case KVM_REG_RISCV_CONFIG_REG(mvendorid):
+		if (reg_val == vcpu->arch.mvendorid)
+			break;
 		if (!vcpu->arch.ran_atleast_once)
 			vcpu->arch.mvendorid = reg_val;
 		else
 			return -EBUSY;
 		break;
 	case KVM_REG_RISCV_CONFIG_REG(marchid):
+		if (reg_val == vcpu->arch.marchid)
+			break;
 		if (!vcpu->arch.ran_atleast_once)
 			vcpu->arch.marchid = reg_val;
 		else
 			return -EBUSY;
 		break;
 	case KVM_REG_RISCV_CONFIG_REG(mimpid):
+		if (reg_val == vcpu->arch.mimpid)
+			break;
 		if (!vcpu->arch.ran_atleast_once)
 			vcpu->arch.mimpid = reg_val;
 		else