FreeBSD boot hangs on qemu-kvm on AMD host

[Michael Tokarev <mjt@tls.msk.ru>]

Hello.

Dominik Klein (Cc'd) reported on irc that current
FreeBSD release hangs at boot in qemu-kvm.  It is
trivially verifiable by d/loading the bootonly
FreeBSD 8.2 CD image from ftp.freebsd.org and
running

  kvm -cdrom FreeBSD-8.2-RELEASE-amd64-bootonly.iso.

The guest unpacks kernel, draws a menu (apparently
in real kernel already, not in the boot loader) and
starts a 10-second countdown.  At this point it just
stops (either at 10 or 9, ie, right at the beginning
of the cycle) and kvm process starts using 100% of
the CPU core given to it.  With this typical kvm_stats
from qemu-kvm 0.14:

kvm statistics

 exits                                     43397783 1400960
 halt_exits                                42862622 1400769
 irq_exits                                     5487     172
 host_state_reload                           300074     101
 io_exits                                    292212      18
 irq_injections                                 768      18
 halt_wakeup                                    257       3
 mmu_pte_write                              7702163       0
 insn_emulation                              378273       0
 mmio_exits                                   15786       0
 pf_fixed                                      4817       0
 mmu_shadow_zapped                              170       0
 mmu_cache_miss                                 151       0
 remote_tlb_flush                                 2       0
 signal_exits                                     1       0
 fpu_reload                                       1       0


I tried i386 and amd64 guest images, and tried the same
thing with qemu-kvm 0.12.5 and 0.14.0 - the effect is
exactly the same.

So far the 3 affected systems where we tried this are all
AMD machines.  My home box is AthlonII, my work box is
PhenomI 9750, and Dominik tries it on Opteron 6174.

Running it with -no-kvm lets the guest to go (but obviously
very slow).  Using -no-kvm-irqchip helps too.

Any guess what it can be?

Thanks!

/mjt