Re: [RFC PATCH] x86/fpu: Don't unconditionally add XFEATURE_MASK_FPSSE on sigentry

[Dave Hansen]

On 4/26/19 12:26 AM, Sebastian Andrzej Siewior wrote:
>> That's just a guess, though.
>>
>> If we care, I think we should just use XSAVE instead of XSAVEOPT and
>> trying to reconstruct the init state in software.
> We can't use XSAVE directly in the slowpath. We need to reconstruct the
> init state. We have the mxcsr quirk. We would need just to extend it and
> set the FP area to init state if the FP state is missing like we do in
> fpstate_sanitize_xstate().

Can you remind me why we can't use XSAVE directly in the slow path?