FreeBSD 12.1 i386 on Hyper-V Fails to Mount Root

[Harry Schmalzbauer]

Am 19.05.2021 um 13:26 schrieb Harry Schmalzbauer:

...
> I can just assure that I tracked it down to the single r332489 committ 
> which makes vmbus(4) failing on i386.
> Verification done:
> - built/installed world&kernel on i386 HyV guest with r332488
>   reboot fine.
> - built/installed kernel on the same vm after svnlite up -r 332489
>   reboot fails with "vmbus0: cannot find free IDT vector"
> - loaded kernel.old/kernel, did revert 332489, by 'svnlite up -r 
> 332488' and compiled/reinstalled kernel (again r332488)
>   reboot fine.

While having hard time catching up with x86 basics, I found another oddity:
On Hyper-V gen1, the maximum ammount of RAM I can assign a FreeBSD guest 
beeing able to boot i386 is 3914MB, startring with r332489.
As soon as I configure the guest to have 3916MB or more, FreeBSD-i386 
r332489 booting leads to guest reset.  Loader starts kernel, machine 
reboots.
Either reverting to r332488 or reducing guest-RAM solves this.

The most interesting fact here is that I couldn't reproduce it on ESXi.
I ducplicated the Hyper-V HDD and started the exactly same code on 
ESXi(v6) guest, where it doesn't crash!  I can boot FreeBSD-i386 r332489 
with any ammount of guest-RAM on ESXi.
(speaking for GENERIC, non-PAE)

Host CPU differs significantly though and I have no possibility to test 
this influence.  Ther HyperV is Dual-Socket E5-2630 while the ESXi host 
is a single socket X3470 Xeon (pre SandyBridge).

Maybe someone has an idea how this affects these symptoms.

Thanks,
-harry