Hi,
> > > The patch makes no difference for booting directly from boot2 ... except
> > > memsize() in boot2 also fails to check for errors, so it returns garbage
> > > values.
> >
> > Yes I know that :-)
> > But booting kernel directly from boot2 is not working at all for
> > several years, so my understanding is that /boot/loader is necessary
> > to boot kernel.
>
> It mostly works. I submitted patches in a PR many years ago and still use
> them.
Great! I tried to find the PR, but couldn't find...
Why isn't it committed yet? :-)
> > Hmmm, actually no. I know that some machines get panic with fatal trap
> > 12 if we do 0x12 call. The worst case is getting panic, not losing
> > 640K memory.
>
> Where does the panic occur? I checked that there is no problem here if
> the result of INT 0x12 is ignored and basemem is set to 0.
panic messages attached. It seems to be within BIOS routine after reti
jumped from vm86_bioscall.
Strangely, this panic can be recovered by continue command in DDB,
FreeBSD continues its boot process after this. So, this panic problem
is not serious for me :)
OTOH, the same problem in boot program is very critical, completely stops
with register dump...
> > And it seems that today's Linux don't have 0x12 calling any more,
> > so I didn't see any problem on the machines.
> >
> > Now I have some ideas on this issue;
> > - 0x15/0xe820 call in getmemsize() to determine base mem size. (But how?)
> > - 0x15/0xe820 call in locore.s before calling init386().
> > - specify the size by loader tunable (e.g. hw.basememsize).
>
> I would first fix all the broken code that doesn't check for errors
> and see if the problem goes away. Then recover any low memory not
> reported by int 0x12 in the int 0x15/0xe820 code in i386/machdep.c,
> like libi386/biosmem.c does it (I think machdep.c intentionally skips
> the low memory, while biosmem.c tries to find it).
Cool. Thanks!
Stopped at 0xf842:
Fatal trap 12: page fault while in kernel mode
fault virtual address = 0xf842
fault code = supervisor read, page not present
instruction pointer = 0x8:0xc03b5108
stack pointer = 0x10:0xc0a68e90
frame pointer = 0x10:0xc0a68e94
code segment = base 0x0,limit 0xfffff, type 0x1b
= DPL 0, press 1, def32 1, gran 1
processor eflags = resume,IOPL = 0
current process = 0 ()
kernel:type 12 trap,code=0
db> t
Fatal trap 12: page fault while in kernel mode
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