qemu riscv, thead c906, Linux boot regression

[LIU Zhiwei]

On 2024/1/24 20:49, 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.
>
> Maybe Christoph or Zhiwei can answer?
>
> qemu command-line:
> qemu-system-riscv64 -nodefaults -nographic -machine virt,acpi=off \
>     -cpu thead-c906 ...

Hi  Björn,

I think it is caused by an mmu extension(named XTheadMaee) not 
implemented on QEMU which is conflicts with Svpbmt, which is the reason 
for error-ta in Linux.

I will try to fix this on QEMU and at the same time give a way to 
implement vendor custom CSR(XTheadMaee is controlled by an CSR named 
mexstatus)  on QEMU.

Thanks,
Zhiwei

>
> Thanks,
> Björn