V6: [PATCH] x86: Install <sys/platform/x86.h> [BZ #26124]

[H.J. Lu]

On Tue, Jun 30, 2020 at 2:46 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu:
>
> > On Mon, Jun 29, 2020 at 9:49 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * H. J. Lu:
> >>
> >> >> One thing I still dislike (sorry) is the asymmetry between the usable
> >> >> and feature checks.  For example, why is there are usability check for
> >> >> VAES, but not for AES?  I believe the reason is that VAES depends on
> >> >> AVX/AVX2, but AES only depends on SSE2.  But even that suggests to me
> >> >> that for 32-bit, there should be a usable gate for that (which is false
> >> >> if SSE2 support has been masked).
> >> >
> >> > CPU with AES must have SSE2.  I don't think we need explicit check
> >> > for SSE2.
> >>
> >> But SSE2 needs operating system support, so we get the usable vs
> >> available distinction this way.  I think.
> >>
> >
> > We don't check if OS supports SSE2.
>
> Isn't this a bug?  Not that it matters much these days, but how did this
> work in the beginning?

For SSE support, there is no straightforward way to detect OS support.
We just assume that the kernel supports SSE.

-- 
H.J.