port-amd64/58085: RFDS mitigations -- Register File Data Sampling

[campbell+netbsd%mumble.net@localhost]

>Number:         58085
>Category:       port-amd64
>Synopsis:       RFDS mitigations -- Register File Data Sampling
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Mar 28 14:10:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, 8, ...
>Organization:
The NetBSDomainbypass Foundation
>Environment:
>Description:
New Spectre-class vulnerability -- we may need to adjust our MDS_LEAVE hotpatch sequence to address it, and maybe apply a firmware update:

https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/register-file-data-sampling.html

OpenBSD change for comparison:

https://freshbsd.org/openbsd/src/commit/ykX9FLsfLUh5VFFk
>How-To-Repeat:
use an affected Intel CPU
>Fix:
Yes, please!