qemu riscv, thead c906, Linux boot regression

[Conor Dooley]

On Wed, Jan 24, 2024 at 01:49:51PM +0100, 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.

The "correct" marchid/mimpid values for the c906 are zero.

I haven't looked into the code at all, so I am "assuming" that it is
being zero intialised at present. Linux applies the errata fixups for
the c906 when archid and impid are both zero - so your patch will avoid
these fixups being applied.
Do you think that perhaps the emulation in QEMU does not support what
the kernel uses once then errata fixups are enabled?

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