Blurry display of guests with kernel 4.10.10

[SteveE]

Below posted as a solution for anyone that encounters a similar issue

I recently had to upgrade from VB 5.0 to 5.1.18 on my LinuxMint 18.1 KDE spin laptop with a HiDPI screen. This was because I'd needed to update the kernel to 4.10.10, but the VB 5.0 kernel modules don't support that kernel version. After the upgrade, everything on screen looked ok (although I had to resize some fonts), except for Virtualbox guests.

Before the upgrade,all of my guest OSs had a scaling factor of 2 applied to the screens, but after the upgrade they appeared much larger on the screen, so I put the scaling factor back to 1, which gave the correct screen size for the guests. Unfortunately all of the guests (win 10 and Ubuntu 16.04) have quite unclear displays, the screen is all a bit blurred, making them uncomfortable to work with. I'd applied the latest guest OS additions to the VMs, tried guest video acceleration support on and off, rebooted the host machine, tried other scaling factors, tried forcing the VM screens to resize to a particular resolution, all made no difference to the problem.

Various searches didn't come up with much in the way of suggestions for Linux hosts. There are discussions about Windows hosts using HiDPI, and a suggestion to disable windows HiDPI support for a particular VM using a desktop shortcut. Not a solution on Linux, but it made me think of the scaling factor set in the KDE System Settings - Display and Monitor app. That was previously set to 1.7, which was necessary to make things look right on screen of the host. I changed that back to 1.0, rebooted, and to my surprise found that the screen on the host was unaffected, everything still readable and not shrunk. Better still, when I start up my VirtualBox guests, they are tiny, but re-applying the virtualbox guest scaling factor of 2 that I had before makes them the right size, and they are no longer blurred.

So, in summary:
4.10.10 kernel (perhaps earlier versions too) changes the behaviour on high DPI screens, meaning that the scaling factor you might previously have applied using KDE system settings is no longer required and causes problems with Virtualbox guests. Go to KDE menu > System Settings > Display and Monitor > Displays > Scale Display, move the slider to set it to 1.0, reboot (log on/off might be sufficient, but system settings say restart). Your VB guests should now look as they did before the kernel upgrade.

Hope this helps someone.

[Gary018]

This answered my problem for virtualbox windows guest. But, I need to move my kde up to 1.7 because everything in kde is so terribly tiny. So, I have fixed virtualbox, but I am stuck with tiny everything else.

Is there a way to have the best of both worlds? If I leave the scaling in kde at one, is there other parameters that I can use to make kde look a normal size?