patch for %gs saving

[David Xu]

----- Original Message -----=20
From: "Peter Wemm" <peter at wemm.org>
To: "David Xu" <davidxu at freebsd.org>
Cc: <freebsd-threads at freebsd.org>
Sent: Friday, April 11, 2003 1:37 PM
Subject: Re: patch for %gs saving=20


> "David Xu" wrote:
> > Here is the patch for kernel to save %gs,
> > it works well on my machine.
> > http://people.freebsd.org/~davidxu/i386_gs.diff
> > Daniel, is this the reason in your libpthread
> > patch that doesn't use getcontext syscall ?
>=20
> To put some background on the issue, there is a reason why we did not
> do this.  %gs is not used by the kernel, so it does not normally need =
to
> be saved and restored on every trap into the kernel.  Setting a =
segment
> register is Really Slow - measured in hundreds of clock cycles.
>=20
> So, we normally only touch %gs when we context switch to a different =
process
> that may have a different %gs.  Or when one of the context syscalls =
wants
> it changed.  We cannot avoid touching %fs because we use it for kernel
> private data.  But if it wasn't for that, we wouldn't be touching
> %fs for regular traps/syscalls/etc either.
>=20
> Bruce Evans understands this better than I do, I would suggest not =
making
> this change without talking about it with him first.
>=20

Yes, I know loading a descriptor is slow, but in real world, such =
optimization
will be lost in real work noise. And setcontext syscall is broken by =
this
optimization, userland will fail to set his context in atomic operation,
set pcb->gs =3D userland_gs does not work, so the cost is obviously, I =
can optimize
my patch to only save and restore %gs at trap and interrupt time,  when =
entering
kernel, I can always zero %gs because kernel does not use it, I think =
this can
reduce clock cycles at context switch, this might be better than current =
code which
loading %gs at every context switch.

> Cheers,
> -Peter
> --
> Peter Wemm - peter at wemm.org; peter at FreeBSD.org; peter at yahoo-inc.com
> "All of this is for nothing if we don't go to the stars" - JMS/B5