Change Proposal Name

Summary

Enable 48bit VA on AARCH64

Owner

Name: Jeremy Linton
Email: jeremy.linton@arm.com
Release notes owner:

Current status

Targeted release: Fedora 26
Last updated: 2016-09-29
Tracker bug: <will be assigned by the Wrangler>

Detailed Description

The current aarch64 kernel is using a 42-bit process virtual address (VA) space. This is fairly limiting, but aarch64 processors also have support for 48-bit VA's. This is actually 1-bit more than the "47-bit" VA's currently used on x86_64. This change is a fairly minor kernel configuration but it has caused problems with mozjs and luajit based projects because they have stored tags in their pointers in the 48th address bit. The upstream mozjs project has been fixed, and there are a set of changes outstanding for luajit. Futher there, have been patches applied to rawhide to support it (bug #1242326, bug #1375305. bug #1375547), but the packages dependent on js185 require a rebuild due to an ABI change.

Benefit to Fedora

Larger aarch64 processes won't be constrained by both the virtual as well as physical limitations of the 42-bit VA currently in use. This actually also helps with things like hugetlb's and potentially provides a performace boost. Lastly, it allows fedora to boot on a class of machines that have the majority of their RAM higher in the address space.

Scope

Proposal owners:

Other developers: N/A (not a System Wide Change)

Release engineering: N/A (not a System Wide Change)
- List of deliverables: N/A (not a System Wide Change)

Policies and guidelines: N/A (not a System Wide Change)

Trademark approval: N/A (not needed for this Change)