Intel Skylake Scalable Platform CPU models that belong to Workstation and HEDT
(Basin Falls) segment (SKL-W/X, family 6, model 85, stepping 4) had reports
of system hangs on reboot when revision 0x2000065 of microcode, that was included
from microcode-20191112 update up to microcode-20200520 update, was applied[1].
In order to address this, microcode update to the newer revision had been
disabled by default on these systems, and the previously published microcode
revision 0x2000064 is used by default for the OS-driven microcode update.

Since revision 0x2006906 (included with the microcode-20200609 release)
it is reported that the issue is no longer present, so the newer microcode
revision is enabled by default now (but can be disabled explicitly; see below).

[1] https://github.com/intel/Intel-Linux-Processor-Microcode-Data-Files/issues/21

For the reference, SHA1 checksums of 06-55-04 microcode files containing
microcode revisions in question are listed below:
 * 06-55-04, revision 0x2000064: 2e405644a145de0f55517b6a9de118eec8ec1e5a
 * 06-55-04, revision 0x2000065: f27f12b9d53f492c297afd856cdbc596786fad23
 * 06-55-04, revision 0x2006906: 5f18f985f6d5ad369b5f6549b7f3ee55acaef967

Please contact your system vendor for a BIOS/firmware update that contains
the latest microcode version.  For the information regarding microcode versions
required for mitigating specific side-channel cache attacks, please refer
to the following knowledge base articles:
 * CVE-2017-5715 ("Spectre"):
   https://access.redhat.com/articles/3436091
 * CVE-2018-3639 ("Speculative Store Bypass"):
   https://access.redhat.com/articles/3540901
 * CVE-2018-3620, CVE-2018-3646 ("L1 Terminal Fault Attack"):
   https://access.redhat.com/articles/3562741
 * CVE-2018-12130, CVE-2018-12126, CVE-2018-12127, and CVE-2019-11091
   ("Microarchitectural Data Sampling"):
   https://access.redhat.com/articles/4138151
 * CVE-2019-0117 (Intel SGX Information Leak),
   CVE-2019-0123 (Intel SGX Privilege Escalation),
   CVE-2019-11135 (TSX Asynchronous Abort),
   CVE-2019-11139 (Voltage Setting Modulation):
   https://access.redhat.com/solutions/2019-microcode-nov
 * CVE-2020-0543 (Special Register Buffer Data Sampling),
   CVE-2020-0548 (Vector Register Data Sampling),
   CVE-2020-0549 (L1D Cache Eviction Sampling):
   https://access.redhat.com/solutions/5142751

The information regarding disabling microcode update is provided below.

To disable usage of the newer microcode revision for a specific kernel
version, please create a file "disallow-intel-06-55-04" inside
/lib/firmware/<kernel_version> directory, run
"/usr/libexec/microcode_ctl/update_ucode" to update firmware directory
used for late microcode updates, and run "dracut -f --kver <kernel_version>"
so initramfs for this kernel version is regenerated, for example:

    touch /lib/firmware/3.10.0-862.9.1/disallow-intel-06-55-04
    /usr/libexec/microcode_ctl/update_ucode
    dracut -f --kver 3.10.0-862.9.1

To disable usage of the newer microcode revision for all kernels, please create
file "/etc/microcode_ctl/ucode_with_caveats/disallow-intel-06-55-04", run
"/usr/libexec/microcode_ctl/update_ucode" to update firmware directories
used for late microcode updates, and run "dracut -f --regenerate-all"
so initramfs images get regenerated, for example:

    mkdir -p /etc/microcode_ctl/ucode_with_caveats
    touch /etc/microcode_ctl/ucode_with_caveats/disallow-intel-06-55-04
    /usr/libexec/microcode_ctl/update_ucode
    dracut -f --regenerate-all

Please refer to /usr/share/doc/microcode_ctl/README.caveats for additional
information.