Re: x86 question ...

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 09/11/2012 12:55 AM, Lars Segerlund wrote:

>  I read that on arm there was a set of patches for using a single
> virtual adress space, ( ie. all processes live in the same mapping so
> to speak , but still protected ), which greatly reduced context
> switching time on that platform, does anyone know if the same would be
> true for x86 ( maily x86_64 ).


If the patch you are talking about is the FCSE patch, no, it will not
work for x86:
- the FCSE patch allows avoiding to flush the cache at every context
switch on armv4 and armv5, because their cache architecture (VIVT)
requires this cache flush otherwise, but x86 have a PIPT cache, so, no
cache flush is needed during the context switches;
- the FCSE patch requires hardware support, the FCSE (Fast Context
Switch Extension) pid register, despite the fact that every process
lives in a separate 32MiB slice below the 3GB limit, its address-space
appears to be [0;32MiB[, this allows "fork" to work; x86, as far as I
know, has no such hardware.

The advantage compared to uCLinux is that you still have memory
protection, but the performances are probably a bit worse because the
TLB is still invalidated at every context switch. So, these are
different trade-off.

-- 
                                                                Gilles.