X fails to start with Guest Additions installed on Fedora 23

[nathanbp]

I'm running Fedora 23 as a guest and after updating at some point, X now fails to start with guest additions installed (it does start if I uninstall the guest additions). My installation is up to date.
Kernel: 4.5.7-200.fc23.x86_64
Virtualbox version: 5.0.24
Guest additions version: 5.0.24

The error in the X log is:
Fatal server error:
[ 5.311] (EE) AddScreen/ScreenInit failed for driver 0

After some Googling, I found a similar Arch Linux issue (issue #48598 on their bug tracker, I can't post a link here since I just registered). Following the suggestion there of adding "iomem=relaxed" to my kernel boot parameters, I am able to start X successfully. However the issue described in that Arch Linux bug was reported fixed in VirtualBox 5.0.18, so this appears to either be a new issue or a regression.

[michael]

If you can reproduce this with a newly installed guest please open a bug report for it, including the full Xorg.0.log file and a log file for the virtual machine. Kernel/system log messages might be interesting too, as that "iomem=relaxed" parameter tells the kernel to disable strict checking of I/O access from user space<1>.

<1> https://lwn.net/Articles/302048/

[michael]

See also bug 15621<1>.

<1> https://www.virtualbox.org/ticket/15621

[a21064]

This seems to happen only if EFI is enabled. X with the guest additions will work fine if EFI is disabled (which is the default) or if EFI is enabled and iomem=relaxed is used as a kernel boot parameter.

I tested Fedora 64 bit guests on Virtualbox 5.1.2, one with and one without EFI.

[michael]

Thank you, that probably explains bug 15683<1>.

<1> https://www.virtualbox.org/ticket/15683