I had the same issue! I am running on ASUS Z390A Prime, with 9900K and RX 580, iGPU in headless mode. Got boot loop after update, disabling iGPU in BIOS solved the boot loop issue. But no more iGPU features.
Then I saw a post to enable H.264 and HEVC on RX 5XX GPUs by running Vega's driver, by adding dummykext144.kext to clover/kexts/others and "shiki-id=Mac-7BA5B2D9E42DDD94 shikigva=32" in config.plist.
Now even I can't get iGPU working, H.264 and HEVC still works. Tested with three 4K HEVC video playing in QuickTime 60fps.
The original post is here (#657):
I try SMBIOS 14.2 / 18.3 , right now I'm on 18.3 Graphics are OK but can not understand HEVC / H264 are work or not. So Video confer show this On 10.14.3, H264 encoder will not work, HEVC encoder work but very unstable. On 10.14.4 beta1, use SMBIOS iMacPro1,1, both AMD H264 / HEVC GVA encoder...
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