> What sort of bottleneck does the EFI stub have so that we need yet
> another envelope?

Well I can come up with a few advantages of UKI compared to normal bzImage with 
builtin initrd and cmdline.

1. You already identified this one. Using addons to adjust your cmdline
2. I can use my normal initramfs generation tooling. Just install my compiled 
kernel, my distros install script will generate the initramfs. Then I package 
it up as a UKI. This will be a lot more awkward with a builtin initrd.
3. Measured boot. You can place PCR signatures in the UKI using 
systemd-measure. This will sign the expected PCR values for booting this UKI. I 
think with normal bzImage this will be a lot more difficult. If you place those 
PCR signatures in the builtin initrd this will change the kernel image which 
means now the values you signed no longer match (depending on how you measure 
the kernel; I don't think the normal EFI stub even measures the kernel in first 
place, but I could be mistaken about this)
4. UKIs are automatically recognized by systemd-boot

There's probably more reasons. The main reason for me to go with UKIs initially 
was the good tooling around them.

You could probably overcome some of these drawbacks in the default kernel EFI 
stub. For example it could also get a place to put signed PCR values. And it 
could also do TPM measurements. However in the process all you're doing is 
rebuilding what already exists today in systemd-stub and the tooling around 
UKIs.

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