Currently guest i915 guess pch type according to processor type in virtual env.
As the real pch isn't in guest, but guest i915 need pch for some function, then guest i915 has to guess the pch type. Normally this guess is right, but this guess may be wrong in the following case:
1) ULT/ULX processor with non LP PCH
2) normal processor with LP pch
In order to fix it, the real pch id should be passed to guest, but there isn't a standard way to do this. Hacker method is needed in QEMU and i915, and kinds of hacker are rejected by QEMU.
This was recognized as a potential issue multiple years ago, but then such strange hardware configurations did not exist even if the potential issue was recognized.
Do you have some specific hardware configuration to specify as an example?
Moving this for the GVT-g team to resolve, as the hypervisor needs to pass the information somehow.