Xenfb2, by extension the Xenfb2 backend in Surfman, will expose default resolution values to the frontend for creating an initial framebuffer:
The default-pitch here is incorrect and should at the minimum be 5120 (1280 * 32 / 8).
Surfman DRM-plugin will try to setup a framebuffer using these values and fail (with drm.debug=0x1e the drm module will also report the error):
Moreover, the default values presumed by the xenfb2 module in the guest do not match either (1024x768 32bpp, so 4096 stride, which is the value used here).
The display does not seem to display any artefact when this fails. The issue is observed in the logs, without debug by the following message:
Discussed in 2019-07-18 monthly call: explore future options for improving and/or using upstream xenfb
As mentioned during the call, here is a starting point on differences between the current xenfb2 and the upstream xenfb.