[Bug 295463] upgrade to 15.0 p9: booting freezes with "EFI framebuffer information"

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=295463

            Bug ID: 295463
           Summary: upgrade to 15.0 p9: booting freezes with "EFI
                    framebuffer information"
           Product: Base System
           Version: 15.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: misc
          Assignee: bugs@FreeBSD.org
          Reporter: bob@vesterman.com

I recently tried to upgrade from 15.0-RELEASE-p8 (which had been working fine)
to 15.0-RELEASE-p9 (which, uh, isn't). I used...

--------
freebsd-update fetch
freebsd-update install
shutdown -r now "Hail Vestertopia!"
--------

... as I have many times in the past for prior upgrades (and as was suggested
(well, maybe not the "Hail Vestertopia!" part) by the security advisory).

The machine now freezes during boot. Specifically, it gets past decrypting the
boot drive (GELI), the boot screen (not sure if that's the right term; I mean
where you can select e.g. multi-user mode or single-user mode), "Loading
kernel...", "Loading configuration modules...", and "Loading splash ok", but
then it freezes after:

--------
EFI framebuffer information:
addr, size     0xde000000, 0x1d4c00
dimensions     800x600
stride         800
masks          0x00ff0000, 0x0000ff00, 0x000000ff, 0x00000000
--------

I waited several minutes (fifteen or so?), and when I came back to it, it was
still frozen at the same point.

I tried ctrl-C, ctrl-D, and ctrl-alt-delete. None seemed to have any effect.

I powered the machine back off and back on.

Got the same result.

Did it again, but this time selected "single user mode" (having used my normal
"multi-user mode" in both earlier reboots).

Got the same result.

Searching for seemingly similar issues, I found various things in the past. One
that seemed of particular interest to my I-Dunno-About-The-Details-Of-Booting
brain was bug #209821, "UEFI - installation media hangs when booting on ASUS
P6P67 DELUXE". Please note that (A) That's not my motherboard, but (B) the
thread contains a bunch of people having the same issue with different
motherboards.

The bug report is "CLOSED - FIXED", and apparently it was fixed in stable/12
with the following explanation (which is beyond my ken):

--------
    efi: loader: Inline copy_finish function in amd64 trampoline

    Instead of calling the efi_copy_finish function from amd64_tramp,
    include the copy instructions in the trampoline code itself.
    This avoids boot hangs and restarts in the cases when
    the efi_copy_finish code happens to be located at a memory
    location that is overwritten during the copy process.

    This is a direct commit to stable/12 since no-copy loader mode requires
    the kernel update not suitable for the branch.
--------

It also mentions there that the issue had been fixed in 13 and in HEAD "some
time ago".

The "tramp" stuff in that message makes me realize that there was something
about "tramp" immediately before the "EFI framebuffer" stuff (which is also
beyond my ken):

--------
Staging 0x6ea00000 (not copying) tramp (0x6e9ff000 PT4 0x6e9f6000)
Start @ 0xffffffff80387000 ...
Loading splash ok
--------

Any help would be appreciated. If there's any additional information I may be
able to provide that might help, please let me know. Thanks.

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