Re: [PATCH v4 10/10] x86: Support compiling out userspace IO (iopl and ioperm)

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Nov 03, 2014 at 12:10:49PM +0000, One Thousand Gnomes wrote:
> On Sun, 2 Nov 2014 09:33:01 -0800
> Josh Triplett <josh@joshtriplett.org> wrote:
> 
> > On the vast majority of modern systems, no processes will use the
> > userspsace IO syscalls, iopl and ioperm.  Add a new config option,
> > CONFIG_X86_IOPORT, to support configuring them out of the kernel
> > entirely.  Most current systems do not run programs using these
> > syscalls, so X86_IOPORT does not depend on EXPERT, though it does still
> > default to y.
> 
> This isn't unreasonable but there are drivers with userspace helpers that
> use iopl/ioperm type functionality where you should be doing a SELECT of
> X86_IOPORT. The one that comes to mind is the uvesa driver. From a quick
> scan it may these days be the only mainstream one that needs the select
> adding.

Should kernel drivers really express dependencies that only their
(current instances of) corresponding userspace components need?
Something seems wrong about that.

> Some X servers for legacy cards still use io port access.

Sure, X servers using UMS rather than KMS seem like a common reason to
need this.

> There are also
> a couple of other highly non-obvious userspace users that hang on for
> some systems - eg some older servers DMI and error records can only by
> read via a real mode BIOS call so management tools have no choice but to
> go the lrmi/io path.

As with any userspace interface, some callers may potentially still
exist.  And this still has "default y", too, to avoid user surprises.

> Still makes sense IMHO.
> 
> From a code perspective however you could define IO_BITMAP_LONGS to 0,
> add an IO_BITMAP_SIZE (defined as LONGS + 1 or 0) and as far as I can see
> gcc would then optimise out a lot of the code you are ifdeffing

IO_BITMAP_LONGS already gets defined to (0/sizeof(long)).  And as far as
I can tell, that would only work for init_tss_io, not anything else.
Even then, that would only work with a zero-size array left around in
tss_struct, which doesn't seem appropriate.  The remaining ifdefs wrap
code that GCC could not constant-fold away, and making that code
constant-foldable seems significantly more invasive than the ifdefs.

- Josh Triplett