[Bug 214718] x86bios panics at boot if it cannot obtain memory

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214718

            Bug ID: 214718
           Summary: x86bios panics at boot if it cannot obtain memory
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: emaste at freebsd.org

Due to an LLD issue (http://llvm.org/pr30891) my kernel was linked at 0x1000,
not 0x200000, meaning that no < 640K memory was available when the system
started up. In this case x86bios panics on boot, because it does an M_WAITOK
allocation when it is not permissible:


ACPI APIC Table: <OVMF   OVMFEDK2>
panic: vm_wait in early boot
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xffffffff81e0ca40
vpanic() at vpanic+0x182/frame 0xffffffff81e0cac0
panic() at panic+0x43/frame 0xffffffff81e0cb20
vm_wait() at vm_wait+0xd6/frame 0xffffffff81e0cb40
kmem_alloc_contig() at kmem_alloc_contig+0x1bd/frame 0xffffffff81e0cc00
contigmalloc() at contigmalloc+0x33/frame 0xffffffff81e0cc40
x86bios_modevent() at x86bios_modevent+0x244/frame 0xffffffff81e0cc60
module_register_init() at module_register_init+0xb0/frame 0xffffffff81e0cc90
mi_startup() at mi_startip+0x118/frame 0xffffffff81e0ccb0
btext() at btext+0x2c
KDB: enter: panic
[ thread pid 0 tid 0 ]
Stopped at      kdb_enter+0x3b: movq    $0,kdb_why

I was quite surprised to find that if I disable x86bios the system boots and
runs successfully in this (linked at 0x1000) configuration.

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