svn commit: r326758 - in head/sys/i386: conf include

[Konstantin Belousov]

On Mon, Dec 11, 2017 at 07:09:10AM -0800, Rodney W. Grimes wrote:
> The current comment about a pcb, I thought that code was changed
> so we only put the pointer to a pcb on the stack.

pcb is on top of the stack, followed by the userspace FPU registers save
area.  I do not see any sense in existence of pcb in modern kernel, it is
a remnant of the user area that was swappable.  Currently pcb is swappable
as well, but the value of this is much less then the overhead we pay by
the stack space reduction.

FPU save area is the on of the problem which makes us increase the amd64
stack size, AVX or even AVX512 make the things much worse.  It is unlikely
that somebody would run 32bit kernel on machine capable of that extensions,
i.e. Haswell or Skylake.