qemu riscv, thead c906, Linux boot regression

[Daniel Henrique Barboza]

On 1/24/24 09:49, Björn Töpel wrote:
> Hi!
> 
> I bumped the RISC-V Linux kernel CI to use qemu 8.2.0, and realized that
> thead c906 didn't boot anymore. Bisection points to commit d6a427e2c0b2
> ("target/riscv/cpu.c: restrict 'marchid' value")
> 
> Reverting that commit, or the hack below solves the boot issue:
> 
> --8<--
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index 8cbfc7e781ad..e18596c8a55a 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -505,6 +505,9 @@ static void rv64_thead_c906_cpu_init(Object *obj)
>       cpu->cfg.ext_xtheadsync = true;
>   
>       cpu->cfg.mvendorid = THEAD_VENDOR_ID;
> +    cpu->cfg.marchid = ((QEMU_VERSION_MAJOR << 16) |
> +                        (QEMU_VERSION_MINOR << 8)  |
> +                        (QEMU_VERSION_MICRO));
>   #ifndef CONFIG_USER_ONLY
>       set_satp_mode_max_supported(cpu, VM_1_10_SV39);
>   #endif
> --8<--
> 
> I'm unsure what the correct qemu way of adding a default value is,
> or if c906 should have a proper marchid.

In case you need to set a 'marchid' different than zero for c906, this hack would
be a proper fix. As mentioned in the commit msg of the patch you mentioned:

"Named CPUs should set 'marchid' to a meaningful value instead, and generic
  CPUs can set to any valid value."

That means that any specific marchid value that the CPU uses must to be set
in its own cpu_init() function.


Thanks,

Daniel


> 
> Maybe Christoph or Zhiwei can answer?
> 
> qemu command-line:
> qemu-system-riscv64 -nodefaults -nographic -machine virt,acpi=off \
>     -cpu thead-c906 ...
> 
> 
> Thanks,
> Björn