From Fedora Project Wiki

Change Proposal Name


Enable 48bit VA on AARCH64


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 and due to the way aarch64 paging works this limits the max physical address as well. This is fairly limiting for some applications, 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.


  • Proposal owners:
  • Other developers: N/A (not a System Wide Change)
  • Release engineering: 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)

Upgrade/compatibility impact

It shouldn't have any noticable impact on upgrades, currently its possible to boot machines with both 42 and 48 bit VA changes simply by picking diffrent kernel options in grub.

How To Test

Normal system operation on aarch64 hardware. This include running package unit tests/etc as normal and watching out for regressions. The change is system wide because it basically affects the entire system, but should be fairly transparent.

N/A (not a System Wide Change)

User Experience

Its unlikely a desktop user would notice the change, except possibly it may boot on additional hardware. A server user could potentially utilize more RAM for in-memory databases/etc.


N/A (not a System Wide Change)

Contingency Plan

In the worse case, the kernel can be reverted back to 42-bit VA's with a single configuration change. Otherwise, we can patch additional mozjs versions (mosjz31 if 0ad's patches arn't complete) or drop the dependent packages from the aarch64 disto until they can be fixed (really this only applies to 0ad at this point).

  • Contingency deadline: Beta freeze
  • Blocks release? No, the likely problem is an additional package is found to have a problem. I would expect that should this happen it won't me a major bug/package as 48-bit VA's have been in test for a while.


As part of this, an attempt has been made to reduce the number of mozjs versions in use. The plan is to move dependent packages to either mozjs24 or mozjs38. There remains some work to move some of the original js 1.8.5 users, but that isn't a critical problem as simply rebuilding them solves the problems caused by the additional address bit.

I can add additional information about the hardware or the dependent projects here if its appropriate.

Release Notes

Enable 48-bit address spaces.