On Wed, 2010-07-07 at 10:03 +0100, Stefan Hajnoczi wrote: > 2010/7/5 Gleb Natapov <gleb@xxxxxxxxxx>: > > On Mon, Jul 05, 2010 at 01:36:08PM +0100, Stefan Hajnoczi wrote: > >> 2010/7/5 Gleb Natapov <gleb@xxxxxxxxxx>: > >> > On Mon, Jul 05, 2010 at 01:11:25PM +0100, Stefan Hajnoczi wrote: > >> >> On Fri, Jul 2, 2010 at 9:07 AM, Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote: > >> >> > On Fri, Jul 2, 2010 at 4:31 AM, ewheeler <kvm@xxxxxxxxxxxxxxx> wrote: > >> >> >> Hello all, > >> >> >> > >> >> >> I'm booting a CentOS kernel under today's KVM git and it hangs after > >> >> >> initializing the serial port when the drive if=virtio, but not when > >> >> >> drive if=ide. Look close---this is not a "forgot to add virtio_blk" > >> >> >> problem. If I use 0.12.3 from Ubuntu 10.04 it works properly. > >> >> >> > >> >> >> Reproduction: > >> >> >> > >> >> >> Using kvm 0.12.3 on ubuntu 10.04 (1:84+dfsg-0ubuntu16+0.12.3+noroms > >> >> >> +0ubuntu9) it will work properly: > >> >> >> > >> >> >> qemu-system-x86_64 -drive file=dummy-disk-image,if=virtio \ > >> >> >> -kernel vmlinuz-2.6.18-194.3.1.el5.centos.plus > >> >> >> > >> >> >> As expected, the kernel panics unable to mount root (good-boot.png). > >> >> >> This makes sense, as "dummy-disk-image" is 1MB of 0x00 bytes. > >> >> >> > >> >> >> ---However---if I use today's git (2010-07-01) of kvm: > >> >> >> > >> >> >> /usr/local/kvm-git/bin/qemu-system-x86_64 -drive file=dummy-disk-image,if=virtio \ > >> >> >> -kernel vmlinuz-2.6.18-194.3.1.el5.centos.plus > >> >> >> > >> >> >> This hangs just after initializing the Serial device (obtained by adding > >> >> >> -serial stdio -append console=ttyS0): > >> >> >> > >> >> >> Note that this only happens with the disk interface set to virtio > >> >> >> (if=virtio). It works fine for ide (if=ide). > >> >> >> > >> >> >> > >> >> >> Am I doing something wrong here? > >> >> >> Is anyone else having this problem? > >> >> > > >> >> > I have seen this issue with a RHEL 5.5 guest running under > >> >> > qemu-kvm.git. It boots a new guest fine but hangs as you described > >> >> > with the RHEL 5.5 kernel. I have not investigated. > >> >> > >> >> This issue is affected by extboot, a feature that enables booting from > >> >> virtio-blk devices. I have just sent a patch to the KVM mailing list > >> >> to restore extboot functionality which has been broken in > >> >> qemu-kvm.git. That patch can be used to work around this issue by > >> >> using "-drive ...,boot=on" but it doesn't explain why the RHEL 5.5 > >> >> kernel hangs during serial initialization when extboot is not present. > >> >> > >> > Hang that happens during guest boot (after bootloader started the > >> > kernel) cannot be worked around by extboot. extboot is also not needed > >> > with latest qemu git to boot from virtio disks since the support for > >> > that is in the bios now. > >> > >> I agree that something else is going on here and needs to be > >> investigated, but I do think that extboot can indirectly affect the > >> guest boot. > >> > >> With extboot the virtio-blk PCI adapter is not touched by the > >> firmware/bootloader. Is it possible that a virtio-blk interrupt is > >> raised and not acknowledged before entering Linux. When Linux brings > >> up the serial port it gets swamped with interrupts? That's just a > >> guess. > >> > > That is possible when bios is actually used to boot the guest, but bug > > reporter uses -kernel option so no bios boot code should run at all. > > Virtio is initialized anyway, but this will happen with boot=on too. > > Okay, I got to the bottom of this. Here's the story, see bottom of the mail for > the solution and workarounds: > > It turns out that -kernel does involve the BIOS. KVM pulls apart the bzImage > and makes it available via the fw_cfg interface. The linuxboot.bin option ROM > is executed by the BIOS inside the VM to actually jump into the kernel. This > means the BIOS does POST and sets itself up; the kernel's real-mode boot > code is going to use BIOS interrupts. > > So now the VM has booted, BIOS finished POST and executed linuxboot.bin, > linuxboot.bin transferred control to Linux. Then, in Linux arch/x86/boot/edd.c > a disk read to sector=0 bytes=512 is made using INT 13h. Since this disk > read comes from the Linux kernel, it happens regardless of -kernel or not. > > The disk read is serviced by the BIOS. Older versions of SeaBIOS leave the > interrupt raised here, so then the kernel hangs in serial initialization later. > > However, there is a simple workaround: > > x86_64-softmmu/qemu-system-x86_64 -m 512 -drive > file=~/rhel5u5.img,if=virtio -kernel /boot/vmlinuz-2.6.32-5-amd64 > -append edd=skipmbr > > When the edd=skipmbr kernel parameter is used, the kernel will not perform > the disk read and the interrupt will never get stuck. The VM boots > successfully. Wow, brilliant. Thank you for jumping into this so quickly! -Eric -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
