We don't have any form of a 'bare bones' CPU. rv64, our default CPUs,
comes with a lot of defaults. This is fine for most regular uses but
it's not suitable when more control of what is actually loaded in the
CPU is required.
A bare-bones CPU would be annoying to deal with if not by profile
support, a way to load a multitude of extensions with a single flag.
Profile support is going to be implemented shortly, so let's add a CPU
for it.
The new 'rv64i' CPU will have only RVI loaded. It is inspired in the
profile specification that dictates, for RVA22U64 [1]:
"RVA22U64 Mandatory Base
RV64I is the mandatory base ISA for RVA22U64"
And so it seems that RV64I is the mandatory base ISA for all profiles
listed in [1], making it an ideal CPU to use with profile support.
rv64i is a CPU of type TYPE_RISCV_BARE_CPU. It has a mix of features
from pre-existent CPUs:
- it allows extensions to be enabled, like generic CPUs;
- it will not inherit extension defaults, like vendor CPUs.
This is the minimum extension set to boot OpenSBI and buildroot using
rv64i:
./build/qemu-system-riscv64 -nographic -M virt \
-cpu rv64i,sv39=true,g=true,c=true,s=true,u=true
Our minimal riscv,isa in this case will be:
# cat /proc/device-tree/cpus/cpu@0/riscv,isa
rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd#
[1] https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc
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: <20231218125334.37184-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
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