On Tue, Jan 19, 2021 at 09:38:59AM -0800, Andy Lutomirski wrote:
> Currently, requesting kernel FPU access doesn't distinguish which parts of
> the extended ("FPU") state are needed. This is nice for simplicity, but
> there are a few cases in which it's suboptimal:
>
> - The vast majority of in-kernel FPU users want XMM/YMM/ZMM state but do
> not use legacy 387 state. These users want MXCSR initialized but don't
> care about the FPU control word. Skipping FNINIT would save time.
> (Empirically, FNINIT is several times slower than LDMXCSR.)
>
> - Code that wants MMX doesn't want or need MXCSR initialized.
> _mmx_memcpy(), for example, can run before CR4.OSFXSR gets set, and
> initializing MXCSR will fail.
>
> - Any future in-kernel users of XFD (eXtended Feature Disable)-capable
> dynamic states will need special handling.
>
> This patch adds a more specific API that allows callers specify exactly
> what they want.
Same nitpicks:
https://lkml.kernel.org/r/20210119110834.gh27...@zn.tnic
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
