Re: [PATCH v4 0/2] MR6225: Populate ARM64 ID register registry keys using the SMBIOS.

[Martin Storsjö]

Martin Storsjö (@mstorsjo) commented about dlls/ntdll/unix/system.c:
> +    snprintf( path_buf, sizeof(path_buf), midr_el1_path, logical_thread_id );
> +    if ((fp = fopen( path_buf, "r" )))
> +    {
> +        fscanf( fp, "%lx", &value );
> +        fclose( fp );
> +        regs[regidx++] = (struct smbios_wine_id_reg_value_arm64){ 0x4000, value };
> +    }
> +
> +#define STR(a) #a
> +#define READ_ID_REG(reg_id) \
> +    /* mrs x0, #reg_id */ \
> +    __asm__ __volatile__( ".inst " STR(0xd5300000 | reg_id << 5) "\n\t" \
> +                          "mov %0, x0" : "=r"(value) :: "x0" ); \
> +    regs[regidx++] = (struct smbios_wine_id_reg_value_arm64){ reg_id, value };
> +
> +    /* Linux traps reads to these ID registers and emulates them. They do not vary across cores,
> FYI, this isn't universally the case - Linux trapping and emulating them hasn't been around since forever.

Linux v4.11 added `HWCAP_CPUID`, and if this hwcap is enabled, the trapping is supported. (I'm not sure if there are kernels past 4.11 that don't have it set, though.) I do run wine on arm64 on a couple of old devboards that run Linux 4.4 kernels - although I haven't updated my version of Wine on them for quite a while...

Anyway, ideally, we'd check `getauxval(AT_HWCAP) & HWCAP_CPUID` before going ahead and trying to read these registers - even if few users today really run older kernels.

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https://gitlab.winehq.org/wine/wine/-/merge_requests/6225#note_81475