[Bug 1789719] Re: severe performance issue after intel-microcode update

[Henrique de Moraes Holschuh]

Basically, *you* need to know if:

1. your workload does a LOT of syscalls (kernel entries from userspace)
OR

2. your workload does a LOT of VMEXITs (runs on a VM) OR

3. uses Intel SGX (you're on your own in that case, go talk to Intel).

Which are directly impacted by the microcode and/or kernel changes.

You can also selectively disable KPTI/PTI, L1D flushes (L1TF
mitigation), and SSBD mitigation to track down which is not compatible
with your workload.  That *might* give enough hints on how to change
your workload to not suffer as much.

https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html

and also

https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html
( read the documentation for  the parameters pti=, spectre_v2=, spec_store_bypass_disable= )

You can disable kernel microcode updating (which will revert to the
microcode in your BIOS/UEFI) using the dis_ucode_ldr parameter on the
*bootloader* (grub, etc).  Instructions are in intel-microcode's README
file in /usr/share/doc/intel-microcode

You can just install an older version of the intel-microcode package and
reboot, if you want to switch to an earlier microcode update **provided
that your system firmware (BIOS/UEFI) does not have a newer microcode**.

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https://bugs.launchpad.net/bugs/1789719

Title:
  severe performance issue after intel-microcode update

Status in intel-microcode package in Ubuntu:
  New

Bug description:
  After upgrading my intel-microcode package to
  3.20180807a.0ubuntu0.16.04.1 the performance for some test code that I
  have has worsened by 400-500%.  Chip is an i7-3960X.

  lsb_release -rd
  Description:	Ubuntu 16.04.5 LTS
  Release:	16.04

  apt-cache policy intel-microcode
  intel-microcode:
    Installed: 3.20180807a.0ubuntu0.16.04.1
    Candidate: 3.20180807a.0ubuntu0.16.04.1
    Version table:
   *** 3.20180807a.0ubuntu0.16.04.1 500
          500 http://us.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
          500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
          100 /var/lib/dpkg/status
       3.20151106.1 500
          500 http://us.archive.ubuntu.com/ubuntu xenial/restricted

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