target/riscv: skip features setup for KVM CPUs

target/riscv: skip features setup for KVM CPUs

As it is today it's not possible to use '-cpu host' if the RISC-V host
has RVH enabled. This is the resulting error:

$ ./qemu/build/qemu-system-riscv64 \
    -machine virt,accel=kvm -m 2G -smp 1 \
    -nographic -snapshot -kernel ./guest_imgs/Image  \
    -initrd ./guest_imgs/rootfs_kvm_riscv64.img \
    -append "earlycon=sbi root=/dev/ram rw" \
    -cpu host
qemu-system-riscv64: H extension requires priv spec 1.12.0

This happens because we're checking for priv spec for all CPUs, and
since we're not setting  env->priv_ver for the 'host' CPU, it's being
default to zero (i.e. PRIV_SPEC_1_10_0).

In reality env->priv_ver does not make sense when running with the KVM
'host' CPU. It's used to gate certain CSRs/extensions during translation
to make them unavailable if the hart declares an older spec version. It
doesn't have any other use. E.g. OpenSBI version 1.2 retrieves the spec
checking if the CSR_MCOUNTEREN, CSR_MCOUNTINHIBIT and CSR_MENVCFG CSRs
are available [1].

'priv_ver' is just one example. We're doing a lot of feature validation
and setup during riscv_cpu_realize() that it doesn't apply to KVM CPUs.
Validating the feature set for those CPUs is a KVM problem that should
be handled in KVM specific code.

The new riscv_cpu_realize_tcg() helper contains all validation logic that
are applicable to TCG CPUs only. riscv_cpu_realize() verifies if we're
running TCG and, if it's the case, proceed with the usual TCG realize()
logic.

[1] lib/sbi/sbi_hart.c, hart_detect_features()

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230706101738.460804-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>