FreeBSD 12.1 i386 on Hyper-V Fails to Mount Root

[Wei Hu]

Hi Harry,

Can you test if this patch fixes the problems?

https://reviews.freebsd.org/D30577

Thanks,
Wei



> -----Original Message-----
> From: Harry Schmalzbauer <freebsd at omnilan.de>
> Sent: Friday, May 21, 2021 8:50 PM
> To: Konstantin Belousov <kostikbel at gmail.com>; Wei Hu <weh at microsoft.com>
> Cc: freebsd-virtualization at freebsd.org
> Subject: Re: FreeBSD 12.1 i386 on Hyper-V Fails to Mount Root
> 
> 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
>