When I try to increase the resolution in my virtual machine (Xen), running a debian distribution (Kali/Parrot in this case), I get a black screen. I use xrandr, because in the GUI, the max resolution is not high enough.
I managed to crank it a bit up, but at a certain level, it just shows a black screen.
The commands I used:
First we use
gtf for calculating the modeline of possible resolutions:
gtf <xres> <yres> <refresh> [-x]
Give the desired values for x-resolution, y-resolution and refresh rate. The flag -x is default and can be omitted. For a portrait resolution 480x640 at 60 Hz you will get the following output:
$ gtf 480 640 60 -x $ 480x640 @ 60.00 Hz (GTF) hsync: 39.78 kHz; pclk: 24.82 MHz Modeline "480x640_60.00" 24.82 480 504 552 624 640 641 644 663 -HSync +Vsync
From this we can copy & paste the modeline to add it to xrandr list of known resolutions:
xrandr --newmode "480x640_60.00" 24.82 480 504 552 624 640 641 644 663 -HSync +Vsync
xrandr --addmode VBOX0 480x640_60.00
VBOX0 with the appropriate monitor.