Hi Kurt,

Is there any reasonable tutorial for using kgdb with the
bvmdebug kernel option?

 Not really (https://wiki.freebsd.org/BHyVe/gdb)

In any event, 9.2 doesn't have bvmdebug, though it would be a simple backport. It's also not strictly required - you can use the serial port same as on h/w; see below.

A couple of folks I know have run into issues trying to
debug a FreeBSD stable/9 kernel from their bhyve
hosting machine (running stable/10).

In particular, the loadable modules that are in use in
the stable/9 kernel are being "troublesome" to get to
the point where source-level debugging actually works.

Even a pointer to a couple of "worked" examples might be
useful.

I've read this:
http://people.freebsd.org/~jhb/papers/bsdcan/2008/article/node4.html
but not all the techniques in there appear to work properly.

 I tried to repro this with some success from a host running CURRENT.

Firstly, I installed a 9.2 VM, with source. I edited GENERIC and added options DDB and GDB, and reinstalled the kernel.

The disk was then copied, and mdconfig'd/mounted on the host to provide access to the just-buit 9.2 kernel syms and sources.

 com2 was set up as a debug port by dropping to the bhyveload prompt and

   hint.uart.1.flags="0x80"

 (this could also have been done in the guest's /boot/loader.conf)

 com2 was then set up in the bhyve command line to point to an nmdm device

  -l com2,/dev/nmdm0A

 The guest probed uart1 as a debug port:

uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 flags 0x80 on acpi0
ioapic0: routing intpin 3 (ISA IRQ 3) to lapic 0 vector 64
uart1: fast interrupt
uart1: debug port (9600,n,8,1)

After booting, I loaded the the tap device in the guest to provide a kld for kgdb to examine:

root@fbsd9-2:~ # kldload if_tap
root@fbsd9-2:~ # kldstat
Id Refs Address            Size     Name
 1    3 0xffffffff80200000 15f92d8  kernel
 2    1 0xffffffff81a12000 59e9     if_tap.ko

 Now time to try kgdb:

root@fbsd9-2:~ # sysctl debug.kdb.enter=1
debug.kdb.enter: 0KDB: enter: sysctl debug.kdb.enter
[ thread pid 577 tid 100054 ]
Stopped at      kdb_enter+0x3b: movq    $0,0xaf0362(%rip)
db> gdb
(ctrl-c will return control to ddb)
Switching to gdb back-end
Switching to gdb back-end

 In another window, cd'd to the mounted copy of the 9.2 disk:

kgdb -r /dev/nmdm0B kernel.debug
...
This GDB was configured as "amd64-marcel-freebsd"...Switching to remote protocol kdb_enter (why=0xffffffff80fbf671 "sysctl", msg=0x80 <Address 0x80 out of bounds>) at ../../../kern/subr_kdb.c:441
441                     kdb_why = KDB_WHY_UNSET;

(kgdb)

After some experimentation, I found the way to get the correct symbols for the kld was to manually specify it:

(kgdb) add-kld /mnt/boot/kernel/if_tap.ko
add symbol table from file "/mnt/boot/kernel/if_tap.ko.symbols" at
        .text_addr = 0xffffffff81a12000
        .rodata.str1.8_addr = 0xffffffff81a13b50
        .rodata.str1.1_addr = 0xffffffff81a13ddb
        set_sysinit_set_addr = 0xffffffff81a13f68
        set_modmetadata_set_addr = 0xffffffff81a13f80
        set_sysctl_set_addr = 0xffffffff81a13f90
        set_sysuninit_set_addr = 0xffffffff81a13fc0
        .data_addr = 0xffffffff81a13fe0
        .bss_addr = 0xffffffff81a14620
(y or n) y

 However, 'info sharedlibrary' didn't seem to reflect this:

(kgdb) info sharedlibrary
From                To                  Syms Read   Shared Object Library
0xffffffff81a12000 0xffffffff81a13c04 No /boot/kernel/if_tap.ko.symbols

Might have been a bug there, since I was able to successfully set breakpoints in if_tap routines and have them trigger.

I did have some trouble getting the source path set up correctly, but never fully investigated that: seems like gdb is quite rich in that area and it should be possible to get sorted.

later,

Peter.



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