floating points in kernel space

julie Sullivan kernelmail.jms at gmail.com
Tue Jan 4 17:19:06 EST 2011

> On Tue, Jan 4, 2011 at 22:44, Dave Hylands <dhylands at gmail.com> wrote:
> > That is correct. In some architectures, attempts to use floating point
> > from the kernel will work. I've seen some x86 code that uses it.
> AFAIK, once x86 didn't supported due to floating point related
> registers are not correctly (or even doing?) saved and restored during
> context switching. So maybe it is fixed now...
> --

I've often wondered about this oft-cited kernel behaviour too, in my
naivety.  I understand
that this must be on a per-arch basis, but does this mean that the kernel
doesn't police FP
access at _all_ (perhaps this is what Mohit means too)? Does code like X for
example have to access it directly, or does it just use the GPU? What about
other user-space code - does it
have a separate library and do its own security? Video drivers?
Sorry if these are basic questions, I grepped for float in the kernel but
as-yet the associated
code looks really arcane to me - if anyone could answer any of these
questions generally
(if that's possible) that would be very helpful with visualizing the
Maybe I'm looking in the wrong place.

When I started looking at the kernel I imagined this small, neat, concise
piece of highly
efficient code so I wasn't surprised there was no float (don't laugh - how
one learns :-/ ) ...
I suppose any float per-arch 'hacks' (to get a larger word size) would not
be worth
the overhead of the mode switch and extra code?

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