Bug 1954582

Summary:	[OSP 16.2] Document Intel TSX guidance. (This affects updates and upgrades.)
Product:	Red Hat OpenStack	Reporter:	Kashyap Chamarthy <kchamart>
Component:	documentation	Assignee:	Vlada Grosu <vgrosu>
Status:	CLOSED NEXTRELEASE	QA Contact:	RHOS Documentation Team <rhos-docs>
Severity:	unspecified	Docs Contact:
Priority:	unspecified
Version:	16.2 (Train)	CC:	dvd, uemit.seren, vgrosu
Target Milestone:	---	Keywords:	Documentation, FutureFeature, Triaged
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2021-07-29 17:27:54 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:
Bug Blocks:	1975448, 1975450

Description Kashyap Chamarthy 2021-04-28 12:18:51 UTC

Problem Description
-------------------

Intel "transactional synchronization Extensions" (TSX) hardware feature
was disabled by default in RHEL-8.3[1] kernel, specifically from
kernel-4.18.0-214.el8 onwards.  This was done to address performance 
penalty of "TAA" (TSX Asynchronous Abort) mitigations on Intel
Cascadelke servers.

However, as an undesirable side-effect, it breaks forward live migration
compatibility of libvirt-based KVM guests — e.g. when migrating a guest
from RHEL-7.9 to RHEL-8.3 (or from RHEL-8.2 to RHEL-8.3).

We need to document this fallout for OSP.  Including the TripleO work 
that is being done in this area[3].  This affects the following
scenarios:

  - Major upgrade from OSP-13 to OSP-16.2

  - Minor _update_ from OSP-16.1 to OSP-16.2

  - New OSP 16.2 deployments


References
----------

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1828642 — kernel: Disable
    Intel TSX by default on newer CPUs 

[2] https://bugzilla.redhat.com/show_bug.cgi?id=1923118 — [kernel]
    "redhat/configs: Change Intel TSX default to off" breaks live
    migration of KVM guests

[3] https://bugzilla.redhat.com/show_bug.cgi?id=1923165 — [OSP-16.2]
    [Upgrades][TripleO] Add a config to disable Intel "TSX" on RHEL-8.3
    kernel

Comment 2 Kashyap Chamarthy 2021-06-18 15:00:37 UTC

This is now taken care of here[1] (but _again_ needs updating -- see further below):

That said, right after publishing the above, Intel came up with a CPU microcode update to disable TSX in the hardware:

    https://access.redhat.com/articles/6101171
    Intel June 2021 Microcode Update

In the above update, in the "Transactional Synchronization Extension (TSX) Deprecation" section, it says:

    "... update disables TSX by default on some platforms"

Which is most likely to mean:

    The TSX flags ('hle' and 'rtm') won't show up in /proc/cpuinfo anymore on hosts that have applied this CPU microcode.


So we need to update the KBase[1] and also OSP documentation potentially...


[1] https://access.redhat.com/solutions/6036141 — Guidance on Intel TSX impact on OpenStack guests (applies for RHEL 8.3 and above)

Comment 3 Vlada Grosu 2021-07-07 09:03:28 UTC

Hi Kashyap,

Thank you for opening this ticket!

I have actually started working on a draft to document the fact that the TSX feature is now disable by default starting with RHEL 8.3: 
 
* in BZ#1975450 (related eng BZ#1975240) for the 'Keeping Red Hat OpenStack Platform Updated' doc 

* and BZ#1975448 for the 'Framework for Upgrades (13 to 16.1)' doc

I will link this BZ with the other two.

Many thanks,
Vlada

Comment 4 Vlada Grosu 2021-07-29 17:27:54 UTC

Closing this ticket as the content changes required for this BZ have been addressed in the following bugs.

Please feel free to reopen the ticket if you think that more changes are necessary for the 16.2 OSP docs.

Thank you


(In reply to Vlada Grosu from comment #3)
> Hi Kashyap,
> 
> Thank you for opening this ticket!
> 
> I have actually started working on a draft to document the fact that the TSX
> feature is now disable by default starting with RHEL 8.3: 
>  
> * in BZ#1975450 (related eng BZ#1975240) for the 'Keeping Red Hat OpenStack
> Platform Updated' doc 
> 
> * and BZ#1975448 for the 'Framework for Upgrades (13 to 16.1)' doc
> 
> I will link this BZ with the other two.
> 
> Many thanks,
> Vlada