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List:       linux-kernel
Subject:    Re: [patch] fastcall-2.3.32-B6, SYSENTER/SYSEXIT support
From:       Peter Samuelson <peter () wire ! cadcamlab ! org>
Date:       1999-12-13 20:16:58
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[Richard Gooch <rgooch@ras.ucalgary.ca>]
> I propose a much simpler abstraction: set up a global page (which
> always appears at a fixed address in user-space), and set up a jump
> table. Have one jump vector per system call. That's the ABI. End of
> story.

I add my <AOL> to those who have already spoken favorably of this....

> I'm happy with the page mapping idea, but what concerns me is that we
> can end up with a kernel which has a fair bit of code data embedded
> in it, due to the increasing number of syscall instructions.

We are still talking ia32 only, right?  Because the SPARC people would
*really* have fun with this sort of proposal....

> Even if it's contained in __init sections, it still bloats the kernel
> image. This is a particular problem with embedded systems. Config
> options will help here, but we have too many of those already.

Two extra options, the way I see it:

  tristate "  Set up new system call interfaces dynamically" CONFIG_NEW_SYSCALLS
  if [ "$CONFIG_NEW_SYSCALLS" != "n" ]; then
     bool "    Optimize only for lowest-common-denomenator CPU type" CONFIG_NEW_SYSCALLS_LCD
  fi

The second would mean if you compile for 586TSC, the setup code only
knows about a 586TSC-compatible jump table -- nothing newer or older.

Red Hat would ship CONFIG_386 and CONFIG_NEW_SYSCALLS_LCD=n.

> 2) have a single module which contains all the code data variants and
>    writes the appropriate selection to the global page(s)

I vote for this, but it is monolithic-capable (__init) as well.

Peter

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