If this option is disabled, you allow userspace (root) access to all
of memory, including kernel and userspace memory. Accidental
access to this is obviously disastrous, but specific access can
be used by people debugging the kernel. Note that with PAT support
enabled, even in this case there are restrictions on /dev/mem
use due to the cache aliasing requirements.

If this option is switched on, the /dev/mem file only allows
userspace access to PCI space and the BIOS code and data regions.
This is sufficient for dosemu and X and all common users of
/dev/mem.

If in doubt, say Y.

Enables the informational output from the decompression stage
(e.g. bzImage) of the boot. If you disable this you will still
see errors. Disable this if you want silent bootup.

Write kernel log output directly into the VGA buffer or to a serial
port.

This is useful for kernel debugging when your machine crashes very
early before the console code is initialized. For normal operation
it is not recommended because it looks ugly and doesn't cooperate
with klogd/syslogd or the X server. You should normally N here,
unless you want to debug such a crash.

Write kernel log output directly into the EHCI debug port.

This is useful for kernel debugging when your machine crashes very
early before the console code is initialized. For normal operation
it is not recommended because it looks ugly and doesn't cooperate
with klogd/syslogd or the X server. You should normally N here,
unless you want to debug such a crash. You need usb debug device.

This option will cause messages to be printed if free stack space
drops below a certain limit.

Say Y here if you want to show the kernel pagetable layout in a
debugfs file. This information is only useful for kernel developers
who are working in architecture specific areas of the kernel.
It is probably not a good idea to enable this feature in a production
kernel.
If in doubt, say "N"

Mark the kernel read-only data as write-protected in the pagetables,
in order to catch accidental (and incorrect) writes to such const
data. This is recommended so that we can catch kernel bugs sooner.
If in doubt, say "Y".

This option enables a testcase for the DEBUG_RODATA
feature as well as for the change_page_attr() infrastructure.
If in doubt, say "N"

This option helps catch unintended modifications to loadable
kernel module's text and read-only data. It also prevents execution
of module data. Such protection may interfere with run-time code
patching and dynamic kernel tracing - and they might also protect
against certain classes of kernel exploits.
If in doubt, say "N".

This option enables a testcase for the CPU NX capability
and the software setup of this feature.
If in doubt, say "N"

This option allows trapping of rare doublefault exceptions that
would otherwise cause a system to silently reboot. Disabling this
option saves about 4k and might cause you much additional grey
hair.

Force the IOMMU to on even when you have less than 4GB of
memory and add debugging code. On overflow always panic. And
allow to enable IOMMU leak tracing. Can be disabled at boot
time with iommu=noforce. This will also enable scatter gather
list merging.  Currently not recommended for production
code. When you use it make sure you have a big enough
IOMMU/AGP aperture.  Most of the options enabled by this can
be set more finegrained using the iommu= command line
options. See Documentation/x86_64/boot-options.txt for more
details.

This option disables various optimizations in IOMMU related
code to do real stress testing of the IOMMU code. This option
will cause a performance drop and should only be enabled for
testing.

Add a simple leak tracer to the IOMMU code. This is useful when you
are debugging a buggy device driver that leaks IOMMU mappings.

Perform x86 instruction decoder selftests at build time.
This option is useful for checking the sanity of x86 instruction
decoder code.
If unsure, say "N".

This is the traditional Linux IO delay used for in/out_p.
It is the most tested hence safest selection here.

Use port 0xed as the IO delay. This frees up port 0x80 which is
often used as a hardware-debug port.

Use udelay(2) as the IO delay method. This provides the delay
while not having any side-effect on the IO port space.

No port-IO delay. Will break on old boxes that require port-IO
delay for certain operations. Should work on most new machines.

This option will cause struct boot_params to be exported via debugfs.

Do change_page_attr() self-tests every 30 seconds.

This option determines if the kernel forces gcc to inline the functions
developers have marked 'inline'. Doing so takes away freedom from gcc to
do what it thinks is best, which is desirable for the gcc 3.x series of
compilers. The gcc 4.x series have a rewritten inlining algorithm and
enabling this option will generate a smaller kernel there. Hopefully
this algorithm is so good that allowing gcc 4.x and above to make the
decision will become the default in the future. Until then this option
is there to test gcc for this.

If unsure, say N.

Enabling this option turns a certain set of sanity checks for user
copy operations into compile time failures.

The copy_from_user() etc checks are there to help test if there
are sufficient security checks on the length argument of
the copy operation, by having gcc prove that the argument is
within bounds.

If unsure, or if you run an older (pre 4.4) gcc, say N.