target/riscv: allow MISA writes as experimental

target/riscv: allow MISA writes as experimental

At this moment, and apparently since ever, we have no way of enabling
RISCV_FEATURE_MISA. This means that all the code from write_misa(), all
the nuts and bolts that handles how to properly write this CSR, has
always been a no-op as well because write_misa() will always exit
earlier.

This seems to be benign in the majority of cases. Booting an Ubuntu
'virt' guest and logging all the calls to 'write_misa' shows that no
writes to MISA CSR was attempted. Writing MISA, i.e. enabling/disabling
RISC-V extensions after the machine is powered on, seems to be a niche
use.

After discussions in the mailing list, most notably in [1], we reached
the consensus that this code is not suited to be exposed to users
because it's not well tested, but at the same time removing it is a bit
extreme because we would like to fix it, and it's easier to do so with
the code available to use instead of fetching it from git log.

The approach taken here is to get rid of RISCV_FEATURE_MISA altogether
and use a new experimental flag called x-misa-w. The default value is
false, meaning that we're keeping the existing behavior of doing nothing
if a write_misa() is attempted. As with any existing experimental flag,
x-misa-w is also a temporary flag that we need to remove once we fix
write_misa().

[1] https://lists.gnu.org/archive/html/qemu-devel/2023-02/msg05092.html

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Weiwei Li<liweiwei@iscas.ac.cn>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230222185205.355361-4-dbarboza@ventanamicro.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>