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

riscv_vcpu_set_isa_ext_single() will prevent any write of isa_ext regs
if the vcpu already started spinning.

But if there's no extension state (enabled/disabled) made by the
userspace, there's no need to -EBUSY out - we can treat the operation as
a no-op.

zicbom/zicboz_block_size, ISA config reg and mvendorid/march/mimpid
already works in a more permissive manner w.r.t userspace writes being a
no-op, so let's do the same with isa_ext writes.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>

Author: danielhb

arch/riscv/kvm/vcpu_onereg.c

@@ -482,6 +482,9 @@ static int riscv_vcpu_set_isa_ext_single(struct kvm_vcpu *vcpu,
 	if (!__riscv_isa_extension_available(NULL, host_isa_ext))
 		return -ENOENT;
 
+	if (reg_val == test_bit(host_isa_ext, vcpu->arch.isa))
+		return 0;
+
 	if (!vcpu->arch.ran_atleast_once) {
 		/*
 		 * All multi-letter extension and a few single letter