[Bug 226974] kernel DSI read trap at boot

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226974

--- Comment #3 from Leandro Lupori <leandro.lupori at gmail.com> ---
After some more investigation, I found that the part of the commit that "broke"
the boot on powerpc64 with FDT (that is the default now) was the
bdomain[BUF_DOMAINS] array.

By simply adding this array in vfs_bio.c in the previous commit I'm able to
reproduce the issue.

On powerpc64 this array has a size of about 520KB, mainly because the default
MAXCPU value for it is 256, which makes the following bufdomain field take a
lot of space:
struct bufqueue bd_subq[MAXCPU + 1]; /* Per-cpu sub queues + global */

If I change bd_subq size to 17, for instance, then I'm able to go further, but
then I get stuck after this message:
usb_needs_explore_all: no devclass
I don't know if this is related to same issue or not.

However, it is still not clear to me why this extra KBs are a problem for
powerpc64. It is probably something related to FDT, because unsetting "usefdt"
in the bootloader makes the system bootable again, even without the change
above.

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