Hi,

after fixing the VM_BUG_ON, my QEMU guest on the Jetson TK1 generally
refuses to boot. Once in a while it does, but quickly gets stuck again.
In one case I found this in the kernel log (never happened again so
far):

[  762.022874] WARNING: CPU: 1 PID: 972 at ../arch/arm/kvm/../../../virt/kvm/arm/vgic.c:1468 kvm_vgic_sync_hwstate+0x314/0x344()
[  762.022884] Modules linked in:
[  762.022902] CPU: 1 PID: 972 Comm: qemu-system-arm Not tainted 3.19.0-rc7-00221-gfd7a168-dirty #13
[  762.022911] Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
[  762.022937] [<c0025d80>] (unwind_backtrace) from [<c00215d8>] (show_stack+0x10/0x14)
[  762.022958] [<c00215d8>] (show_stack) from [<c065f494>] (dump_stack+0x98/0xd8)
[  762.022976] [<c065f494>] (dump_stack) from [<c0035f38>] (warn_slowpath_common+0x80/0xb0)
[  762.022991] [<c0035f38>] (warn_slowpath_common) from [<c0036004>] (warn_slowpath_null+0x1c/0x24)
[  762.023007] [<c0036004>] (warn_slowpath_null) from [<c001c3c4>] (kvm_vgic_sync_hwstate+0x314/0x344)
[  762.023024] [<c001c3c4>] (kvm_vgic_sync_hwstate) from [<c00147c0>] (kvm_arch_vcpu_ioctl_run+0x210/0x400)
[  762.023041] [<c00147c0>] (kvm_arch_vcpu_ioctl_run) from [<c001063c>] (kvm_vcpu_ioctl+0x2e4/0x6ec)
[  762.023059] [<c001063c>] (kvm_vcpu_ioctl) from [<c01098a4>] (do_vfs_ioctl+0x40c/0x600)
[  762.023076] [<c01098a4>] (do_vfs_ioctl) from [<c0109acc>] (SyS_ioctl+0x34/0x5c)
[  762.023091] [<c0109acc>] (SyS_ioctl) from [<c001e320>] (ret_fast_syscall+0x0/0x34)


BTW, KVM tracing support on ARM seems like it requires some care. E.g.:
kvm_exit does not report an exit reason. The in-kernel vgic also seems
to lack instrumentation. Unfortunate. Tracing is usually the first stop
when KVM is stuck on a guest.

Jan