Re: [PATCH v9 10/26] x86/fpu/xstate: Update the XSTATE buffer address finder to support dynamic states

Bae, Chang Seok
2021-08-18T19:47:19Z

On Aug 18, 2021, at 04:33, Borislav Petkov <bp@alien8.de> wrote:
> On Fri, Jul 30, 2021 at 07:59:41AM -0700, Chang S. Bae wrote:
>>
>> + * @feature_nr: The feature number
>> + *
>> + * Returns: The offset value
>> + */
>> +static unsigned int get_xstate_comp_offset(u64 mask, int feature_nr)
>> +{
>> + u64 xmask = BIT_ULL(feature_nr + 1) - 1;
>> + unsigned int next_offset, offset = 0;
>> + int i;
>> +
>> + if ((xfeatures_mask_all & xmask) == (mask & xmask))
>> + return xstate_comp_offsets[feature_nr];
>> +
>> + /*
>> + * With the given mask, no relevant size is found. Calculate it by
>> + * summing up each state size.
>> + */
>> + for (next_offset = FXSAVE_SIZE + XSAVE_HDR_SIZE, i = FIRST_EXTENDED_XFEATURE;
>> + i <= feature_nr; i++) {
>> + if (!(mask & BIT_ULL(i)))
>> + continue;
>> +
>> + offset = xstate_aligns[i] ? ALIGN(next_offset, 64) : next_offset;
>> + next_offset += xstate_sizes[i];
>
> Why is this more complex than it has to be?
>
> IOW, why can't you simply do:
>
> offset = FXSAVE_SIZE + XSAVE_HDR_SIZE;
> for (i = FIRST_EXTENDED_XFEATURE; i <= feature_nr; i++) {
> if (!(mask & BIT_ULL(i)))
> continue;
>
> if (xstate_aligns[i])
> offset = ALIGN(offset, 64);
>
> offset += xstate_sizes[i];
> }
> return offset;
>
> like it was before?

It was refactored to use in the new helper to find feature_nr’s start point.
If the size is added up here, it is not ‘i’'s start point anymore.

Thanks,
Chang