The mitigation for Meltdown created a new part in the kernel which PatchGuard left unprotected, making hooking of system calls and interrupts possible, even with HVCI enabled.
The world has been informed of Spectre and Meltdown. Spectre and Meltdown are hardware flaws and/or vulnerabilities that allow an attacker to run unprivileged code to read from privileged memory. Due to Spectre and Meltdown being hardware flaws there are alarms and red flags running wild. Three things to consider to block malware from