FYI, we noticed the below changes on

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git virtio_dma
commit 46d2219cb027a0ccee90b980b26a1bb46d186a87 ("virtio_ring: Support DMA APIs")


+----------------------+------------+------------+
|                      | a2b5cd8102 | 46d2219cb0 |
+----------------------+------------+------------+
| boot_successes       | 15         | 0          |
| boot_failures        | 0          | 14         |
| BUG:kernel_boot_hang | 0          | 14         |
+----------------------+------------+------------+


[    1.295643] blk_update_request: I/O error, dev vda, sector 0
[    1.297189] Buffer I/O error on dev vda, logical block 0, async page read
[    2.134088] tsc: Refined TSC clocksource calibration: 2693.503 MHz
[    2.136924] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x26d3451f606, max_idle_ns: 440795333933 ns

Elapsed time: 340
BUG: kernel boot hang
qemu-system-x86_64 -enable-kvm -cpu SandyBridge -kernel /pkg/linux/x86_64-rhel/gcc-4.9/46d2219cb027a0ccee90b980b26a1bb46d186a87/vmlinuz-4.3.0-rc7-00002-g46d2219 -append 'root=/dev/ram0 user=lkp job=/lkp/scheduled/vm-kbuild-yocto-x86_64-40/bisect_boot-1-yocto-minimal-x86_64.cgz-x86_64-rhel-46d2219cb027a0ccee90b980b26a1bb46d186a87-20151028-61392-107kks6-0.yaml ARCH=x86_64 kconfig=x86_64-rhel branch=linux-devel/devel-hourly-2015102814 commit=46d2219cb027a0ccee90b980b26a1bb46d186a87 BOOT_IMAGE=/pkg/linux/x86_64-rhel/gcc-4.9/46d2219cb027a0ccee90b980b26a1bb46d186a87/vmlinuz-4.3.0-rc7-00002-g46d2219 max_uptime=600 RESULT_ROOT=/result/boot/1/vm-kbuild-yocto-x86_64/yocto-minimal-x86_64.cgz/x86_64-rhel/gcc-4.9/46d2219cb027a0ccee90b980b26a1bb46d186a87/0 LKP_SERVER=inn earlyprintk=ttyS0,115200 systemd.log_level=err debug apic=debug sysrq_always_enabled rcupdate.rcu_cpu_stall_timeout=100 panic=-1 softlockup_panic=1 nmi_watchdog=panic oops=panic load_ramdisk=2 prompt_ramdisk=0 console=ttyS0,115200 console=tty0 vga=normal rw ip=::::vm-kbuild-yocto-x86_64-40::dhcp drbd.minor_count=8'  -initrd /fs/sdc1/initrd-vm-kbuild-yocto-x86_64-40 -m 320 -smp 1 -device e1000,netdev=net0 -netdev user,id=net0 -boot order=nc -no-reboot -watchdog i6300esb -rtc base=localtime -drive file=/fs/sdc1/disk0-vm-kbuild-yocto-x86_64-40,media=disk,if=virtio -pidfile /dev/shm/kboot/pid-vm-kbuild-yocto-x86_64-40 -serial file:/dev/shm/kboot/serial-vm-kbuild-yocto-x86_64-40 -daemonize -display none -monitor null 

Thanks,
Ying Huang