Hi
Just got it working today. I have two Intel® Server Board S1200SPL and all you need to do is update the BIOS/Firmware. I used this one Download Intel® Server Board S1200SP BIOS and Firmware Update Package for EFI - If you look at the release note for the BIOS 03.01.D0041, we can also confirm that disable VT for Directed I/O solved the problem.
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KNOWN ISSUES/WORKAROUND
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FEATURES ADDED/REMOVED
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03.01.R0042
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1. Production Sign
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03.01.D0042
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1. [HSD-ES][1507017816]PSIRT - FW-UEFI-Vuln-2018-038 BIOS does not set TCO_BASE_LOCK
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03.01.D0041
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1. [HSD-ES][1507017816]PSIRT - FW-UEFI-Vuln-2018-038 BIOS does not set TCO_BASE_LOCK
2. [HSD-ES][2103624154]Boot into ESXi6.7u1 after enable "VT for Directed I/O", system would hang with purple srcreen.
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Follow this guide: Demo for Performing Firmware Update on EFI* Shell Intel® Server Board... and you will be up and running in no time, with VT-D enabled! :-)