target/riscv/tcg/tcg-cpu.c: consider MISA bit choice in implied rule

target/riscv/tcg/tcg-cpu.c: consider MISA bit choice in implied rule

Gitlab issue [1] reports a misleading error when trying to run a 'rv64'
cpu with 'zfinx' and without 'f':

$ ./build/qemu-system-riscv64 -nographic -M virt -cpu rv64,zfinx=true,f=false
qemu-system-riscv64: Zfinx cannot be supported together with F extension

The user explicitly disabled F and the error message mentions a conflict
with Zfinx and F.

The problem isn't the error reporting, but the logic used when applying
the implied ZFA rule that enables RVF unconditionally, without honoring
user choice (i.e. keep F disabled).

Change cpu_enable_implied_rule() to check if the user deliberately
disabled a MISA bit. In this case we shouldn't either re-enable the bit
nor apply any implied rules related to it.

After this change the error message now shows:

$ ./build/qemu-system-riscv64 -nographic -M virt -cpu rv64,zfinx=true,f=false
qemu-system-riscv64: Zfa extension requires F extension

Disabling 'zfa':

$ ./build/qemu-system-riscv64 -nographic -M virt -cpu rv64,zfinx=true,f=false,zfa=false
qemu-system-riscv64: D extension requires F extension

And finally after disabling 'd':

$ ./build/qemu-system-riscv64 -nographic -M virt -cpu rv64,zfinx=true,f=false,zfa=false,d=false
(OpenSBI boots ...)

[1] https://gitlab.com/qemu-project/qemu/-/issues/2486

Cc: Frank Chang <frank.chang@sifive.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2486
Fixes: 047da861f9 ("target/riscv: Introduce extension implied rule helpers")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240824173338.316666-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>