Doubt Regarding Floating Point Arithmetic

[Prasad Ram]

Thanks @Peter a very good explanation and it's very help full to me.


On 29 July 2014 19:49, Peter Teoh <htmldeveloper at gmail.com> wrote:

> 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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>>
>>
>
>
> --
> Regards,
> Peter Teoh
>
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