Re: [PATCH v9 11/26] x86/fpu/xstate: Update the XSTATE context copy function to support dynamic states

[Bae, Chang Seok]

On Aug 18, 2021, at 05:03, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Fri, Jul 30, 2021 at 07:59:42AM -0700, Chang S. Bae wrote:
>> 
>> -			copy_feature(header.xfeatures & BIT_ULL(i), &to,
>> -				     __raw_xsave_addr(&tsk->thread.fpu, i),
>> -				     __raw_xsave_addr(NULL, i),
>> -				     xstate_sizes[i]);
>> +			unsigned int size = xstate_sizes[i];
>> +			void *from = NULL;
>> +
>> +			/*
>> +			 * Copy the xstate if available. Otherwise, copy the
>> +			 * non-zero init states for legacy states (FP and
>> +			 * SSE) or fill zeros.
>> +			 */
>> +
>> +			if (header.xfeatures & mask)
>> +				from = __raw_xsave_addr(&tsk->thread.fpu, i);
>> +			else if (XFEATURE_MASK_FPSSE & mask)
> 
> The i loop variable above starts from FIRST_EXTENDED_XFEATURE - why is
> this XFEATURE_MASK_FPSSE check even here?

!(header.xfeatures & mask) means init-state should be copied. Except for
these, the init value is zero (as also noted here [1]). So, check this to copy
correct init data if the current iteration is for the legacy states.

At least, I may need to improve the readability here.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/xstate.c#n416

Thanks,
Chang