The i7-870 is a Lynnfield CPU and won't be able to run Fusion 11 or 10.
Thus, the last Fusion version your iMac 27 will run is Fusion 8.5.x. Officially, Fusion 8.5.x is only supported up to Sierra. High Sierra requires Fusion 10.x or later.
You need to talk or email to VMware sales/customer service and ask if they will grant you a downgrade licence. Talk to them BEFORE you make any licence purchase.
Thanks for your prompt reply
"The i7-870 is a Lynnfield CPU and won't be able to run Fusion 11 or 10."
A. What is the reason behind? some bugs in the Lynnfield CPU, or the chipset, or the BIOS, or what?
B. Tested and won't run?
C. Or not tested, but may run?
It will not run as Lynnfield CPUs are part of the Intel Nehalem microarchitecture.
With Fusion 10.x/11.x, an Intel VT-x feature called "VMX Unrestricted Guest" is now required (before it was optional). This feature is available from Westmere microarchitecture (the one after Nehalem) and newer CPUs.
Thanks a lot for your prompt reply.
Guess I will try VirtualBox from Oracle, which has no virtual RAM limitation. I was thinking about Parallels Desktop, but the basic edition just supports max 8GB vRAM per guest VM, and the Pro version is expensive and subscription based.