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SubjectRe: [PATCH v4 18/22] x86/fpu/amx: Define AMX state components and have it used for boot-time checks
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On Mar 20, 2021, at 14:31, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Sun, Feb 21 2021 at 10:56, Chang S. Bae wrote:
>>
>> +static void check_xtile_data_against_struct(int size)
>> +{
>> + u32 max_palid, palid, state_size;
>> + u32 eax, ebx, ecx, edx;
>> + u16 max_tile;
>> +
>> + /*
>> + * Check the maximum palette id:
>> + * eax: the highest numbered palette subleaf.
>> + */
>> + cpuid_count(TILE_CPUID, 0, &max_palid, &ebx, &ecx, &edx);
>> +
>> + /*
>> + * Cross-check each tile size and find the maximum
>> + * number of supported tiles.
>> + */
>> + for (palid = 1, max_tile = 0; palid <= max_palid; palid++) {
>> + u16 tile_size, max;
>> +
>> + /*
>> + * Check the tile size info:
>> + * eax[31:16]: bytes per title
>> + * ebx[31:16]: the max names (or max number of tiles)
>> + */
>> + cpuid_count(TILE_CPUID, palid, &eax, &ebx, &edx, &edx);
>> + tile_size = eax >> 16;
>> + max = ebx >> 16;
>> +
>> + if (WARN_ONCE(tile_size != sizeof(struct xtile_data),
>> + "%s: struct is %zu bytes, cpu xtile %d bytes\n",
>> + __stringify(XFEATURE_XTILE_DATA),
>> + sizeof(struct xtile_data), tile_size))
>> + __xstate_dump_leaves();
>> +
>> + if (max > max_tile)
>> + max_tile = max;
>> + }
>> +
>> + state_size = sizeof(struct xtile_data) * max_tile;
>> + if (WARN_ONCE(size != state_size,
>> + "%s: calculated size is %u bytes, cpu state %d bytes\n",
>> + __stringify(XFEATURE_XTILE_DATA), state_size, size))
>> + __xstate_dump_leaves();
>
> So we have 2 warnings which complain about inconsistent state and that's
> it? Why has this absolutely no consequences? We just keep stuff enabled
> and jug along, right?
>
> Which one of the two states is correct? Why don't we just disable that
> muck and be done with it to play it safe?
>
> Failing to execute some workload by saying NO due to inconsistency is
> far more useful than taking the chance of potential silent data
> corruption.

This change in fact follows the mainline code [1], where this type of warning
is emitted with such mismatch.

Yes, disabling the feature looks to be the right way. Or, perhaps, taking a
large one is an option when mismatched ?

At least, given the feedback, the mainline needs to be revised before applying
this. Correct me if you don’t think so.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/xstate.c#n567

Thanks,
Chang
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