Created attachment 206506 [details]
Screen shot showing CPU type selections are now 32-bit only
H/W: AMD Ryzen 7 3800X CPU, MSI MEG X570 ACE MB (no option to enable or disable virtualization)
Relevant DMESG line:
AMD Virtualization Reference:
'The CPU flag for AMD-V is "svm". This may be checked in BSD derivatives via dmesg or sysctl and in Linux via /proc/cpuinfo.'
Previously created 64-bit VBox machines on earlier H/W and VBox are no longer "defined" as 64-bit machines. All machine type selections are now 32-bit only (see 'FBSD-VBox-5.2.18_no_64_bit.png' screen shot).
Attempting to start a previously configured 64-bit VM now results in an error message (see 'FBSD-VBox-5.2.18_no_VTx_AMD-V_HW.png' screen shot) which further informs that CPU virtualization support is "missing".
Created attachment 206507 [details]
VBox machine startup error message about no recognizable virtualization support
Could not find any relevant documentation at VirtualBox.org about CPU virtualization support or detection.
Created attachment 206523 [details]
MSI MEG X570 ACE BIOS settings relevant to virtulization
Two bits of information that I didn't have before:
a) AMD virtualization (AMD-V) is also referred to as "SVM"
b) MB virtualization settings are in Overclocking area which I skimmed over without making any changes upon initial system installation
Needed to enable IOMMU and SVM Mode
Created attachment 206525 [details]
Verification of MSI MB virtualization controls enabled
Exiting from BIOS setup showing settings changes ("Global C-state" can be ignored)
Created attachment 206526 [details]
OS boot messages verifying AMD SVM enabled
FBSD boot messages showing expanded list of AMD CPU features now including expanded SVM list.
Created attachment 206528 [details]
Additional verification of SVM ("Intel Secure Ring")
More OS boot messages indicating active AMD SVM
So, I now have AMD-V (SVM) enabled and VirtualBox-5.2.18 recognizes 64-bit VM. So this PR can be closed.
VirtualBox Machine/Session Information now shows "VT-x/AMD-V" as Active.
One last clue that pointed me in the right direction came from,
[Solved] No virtualization support with Gigabyte GA-AB350 Gaming 3?