Re: Windows 11 22H2 with passed-through PCI devices hangs in vm_handle_rendezvous() at boot

From: Robert Crowston <>
Date: Wed, 04 Jan 2023 23:55:14 UTC
So it looks like the problem is:

- in one thread we call vcpu_set_state_locked() [from a VM_MAP_PPTDEV_MMIO call from userspace]
-- both the new and old states are VCPU_FROZEN
-- the threads enters a loop while vcpu->state != VCPU_IDLE
-- it gets stuck here forever since nothing will ever change the state to VCPU_IDLE
-- apparently this is to stop two ioctl()s acting on the same vCPU simultaneously, but I don't see any other ioctl against the vCPU in kgdb.

- in all the other threads, we sit in vm_handle_rendezvous()
-- these threads are waiting for the rendezvous to complete
-- every vCPU has completed the rendezvous except for the one stuck in vcpu_set_state_locked()

I see a lot of commits in -CURRENT since my cut of -STABLE, but nothing that looks too relevant. I'll try against CURRENT next.

    — RHC.

------- Original Message -------
On Tuesday, January 3rd, 2023 at 23:54, Robert Crowston <> wrote:

> Still investigating this. AMD 1700, FreeBSD 13.1 stable@3dd6497894. VM is Windows 11 22H2.
> It happens on the setup disk -- at the TianoCore logo, before the "ring" has finished its first rotation -- so very early in the boot process. It's eventually happened for every Win 11 install I have made. Removing the passthrough devices and installing Windows, then re-adding the devices, a fresh install will boot with the passthrough devices a few times, but then shows the same hang behaviour forever after. Windows Boot Repair also hangs. On the host, bhyvectl --destroy hangs. gdb cannot stop bhyve and just hangs as well. None of these hangs show any CPU use. kldunload vmm crashes the host with a page fault. Only a reboot of the host will kill the guest.
> Setting the guest cpu count to 1, or removing all the passthrough devices allows Windows 11 to boot. The same behaviour happens for two different USB controllers I have and two different GPUs. The same bhyve configurations reliably boot Windows Server 2022 and Windows 10 with passthrough working.
> Debugging in userspace, I can see that Windows 11 does PCI enumeration in parallel across multiple cores, and sometimes during boot one vCPU writes a PCI config register at approximately the same time as another vCPU reads that exact register. The hang seems to be aligned with this synchronized write/read. Also, I can sometimes boot successfully under gdb when single stepping PCI cfg register writes, but it's difficult to be sure because my debugging is probably disturbing the timing. I looked at the bhyve code and I don't see what here could be racing in user space. In any event, it's a kernel-side bug.
> Spinning up the kernel debugger, what I always see is:
> 1. 1 bhyve thread in vioapic_mmio_write() -> ... -> vm_handle_rendezvous() -> _sleep()
> 2. 1 bhyve thread in vcpu_lock_one() -> ... -> vcpu_set_state_locked() -> msleep_spin_sbt()
> 3. All remaining bhyve threads, if any, in vm_run() -> vm_handle_rendezvous() -> _sleep().
> Example backtrace attached.
> So it looks like we have some kind of a deadlock between vcpu_lock_one() and vioapci_mmio_write()? Anyone seen anything like it?
>     — RHC.