qemu riscv, thead c906, Linux boot regression

[Björn Töpel]

Daniel Henrique Barboza <dbarboza at ventanamicro.com> writes:

> 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.

Got it. Thanks, Daniel!

For completeness (since it came up on the weekly PW call); Conor pointed
out that zero *is* indeed the right marchid for c906, and in fact, the
non-zero marchid pre commit d6a427e2c0b2 was incorrect.

Post commit d6a427e2c0b2, the correct alternative is picked up, and
ERRATA_THEAD_PBMT (using non-standard memory type bits in
page-table-entries) kicks in. AFAIU, that's not implemented by qemu's
c906 support, which then traps.

That's the theory. Maybe Christoph knows if the non-standard bits are
implemented or not?

Regardless; I removed booting Qemu T-head c906 from the CI, and the
build/boot passes nicely ;-) [1]


Björn

[1] https://github.com/linux-riscv/linux-riscv/actions/runs/7641764759/job/20819801235?pr=447