Doubt Regarding Floating Point Arithmetic

[Peter Teoh]

Perhaps a little explanation:    anything that can be done at userspace,
should not be done at the kernel, simply because doing at the kernel
entailed a lot of security privileges being available.   (ie, logic which
require hardware interaction / access, process scheduling logic or anything
cutting across processes, sharing of common resources like memory etc)
floating point arithmetics is a good example which is not necessary to be
done in the kernel.   Lots of hardware registers are available for FPU
stuff (SSE/SSE2/XMM registers etc):

http://en.wikipedia.org/wiki/SSE2
http://www.godevtool.com/TestbugHelp/XMMintins.htm
http://x86.renejeschke.de/html/file_module_x86_id_117.html

and generally their usage entailed a lot of performance hits when used
extensively (another good reason to avoid it).   And more importantly,
context switching as  provided by Intel processor, the hardware operation
does not include the floating pointers registers (simply because there are
so many of them, and XMM can be like 128 bytes long?)   Context switching
will swap out the entire registers set when switching from one process to
another, and if u were to do this for all the process, when 99% of the time
floating point are not in use, it is a terrible waste of CPU cycle.

Userspace can only interact with the kernel through well-defined syscall -
for purpose of security, interprocess, or hardware access etc.   So
generally it is not possible to schedule floating point instruction (or any
user-defined instructions for that matter) to be executed in the kernel.

But it is possible to schedule floating point arithmetics to be executed in
the kernel indirectly, for example, when u have a special hardware like DSP
that does floating point arithmetics, and u wrote a driver to schedule
instructions to be executed in that hardware unit.  And u have to worry
about many processes concurrently sending instructions to the same unit as
well.

Thanks for the reading.



On Wed, Jul 23, 2014 at 11:15 AM, me storage <me.storage126 at gmail.com>
wrote:

> Hi
> I am reading LDD .In that i didn't understand one point .In Chapter
> 2(Building and Running Modules) they mentioned that
>  " Kernel code cannot do floating point arithmetic"
> .My doubt is which code is used for floating point arithmetic that means
> at low level?
>
> Thank you
>
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> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
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>


-- 
Regards,
Peter Teoh
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