View source for Bogon kernel command line options

'''[[Dankblog|dankblog!]] 2022-03-12, 1842 EST, at [[Viewpoint|the danktower]]'''

i was looking at my dmesg output today, wanting to know something about [[IOMMU]], and noticed the following line:

<tt>[    0.015134] Unknown kernel command line parameters "BOOT_IMAGE=/boot/vmlinuz-5.16.13nlb2 mem_encrypt=off intel_iommu=on", will be passed to user space
</tt>

first off, some basic [https://man7.org/linux/man-pages/man7/kernel-command-line.7.html kernel command line] facts:
<ul>
<li>there's a man page, of course: <tt>kernel-command-line(7)</tt>. many options are explained in <tt>[https://man7.org/linux/man-pages/man7/bootparam.7.html bootparam(7)]</tt></li>
<li>an option is prefixed with <tt>modname.</tt> if it's intended for a module (whether that code has been built as a module, or built into the kernel)</li>
<li>options can be specified in <tt>modprobe.d(5)</tt>, but this only works if the code was actually built as a module</li>
<li>any unrecognized parameter, and any parameter following "--", is passed to <tt>init</tt></li>
<li>parameters containing a "=" go into <tt>init</tt>'s environment</li>
<li>hyphens and underscores are equivalent</li>
</ul>

but unrecognized items get this informational line. let's take a look at them:

<ul>
<li><tt>BOOT_IMAGE=/boot/vmlinuz-5.16.13nlb2</tt> this is set by [https://www.gnu.org/software/grub/ GRUB] since 1.91, released 2005-10-15. i'm not sure why, or who consumes it. LILO set this parameter as well, and this might have just been a compatibility thing, so it might be lost in the river of time.</li>
<li><tt>mem_encrypt=off</tt> this refers to AMD's [https://en.wikichip.org/wiki/x86/sme Secure Memory Encryption] extension, allowing per-page transparent encryption. i've got this disabled in my kernel config now (<tt>CONFIG_AMD_MEM_ENCRYPT</tt>), so it's no longer recognized. i've removed it.</li>
<li><tt>intel_iommu=on</tt> well duh i don't have an intel in [[Schwarzgerat_III|this machine]]. i want <tt>iommu=pt iommu=1</tt></li>
</ul>

the rest of my command line appears to be valid (inspected at <tt>/proc/cmdline</tt>):

<tt>BOOT_IMAGE=/boot/vmlinuz-5.16.13nlb2 root=UUID=939be09a-b63d-403c-996a-8b9866313fbb ro mem_encrypt=off psi=0 acpi_enforce_resources=lax intel_iommu=on</tt>

<tt>psi=0</tt> refers to the Pressure Stall Information feedback system introduced in 2018 by Facebook hackers, which you can read about [https://www.kernel.org/doc/html/latest/accounting/psi.html here] or [https://lwn.net/Articles/764761/ here]. <tt>acpi_enforce_resources=lex</tt> is sadly necessary to control my bling-ass LEDs using [https://openrgb.org/ OpenRGB].

anyway, i wasn't aware of this dmesg output (added on [https://lore.kernel.org/lkml/20210511211009.42259-1-ahalaney@redhat.com/ 2021-05]), but it seems a useful bit of diagnostics. unfuck your kernel command line!

the only crappy element is the low visibility. ideally, a systemd oneshot unit would deliver a potent shock to the operator when this message was detected. constant vigilance!

'''previously: "[[On_the_Effectiveness_of_nonnull|on the effectiveness of nonnull]]" 2021-11-05'''

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