Hi all,
(Sent to cross-distro with debian-arm on cc.)
We have an 'interesting' situation ahead of us, or indeed some of us have already fallen into it:
ARM64 platforms with > 512GB between the lowest and highest RAM addresses end up getting their amount of usable memory truncated if the kernel is built for 39-bit VA (which is what currently happens for Debian kernels). For 4.7, the arm64 defconfig was changed to enable 48-bit VA by default.
While itself not a critical error (but really annoying), in combination with GRUB putting the initrd near the top of available RAM, we end up with systems not booting. We think we've also seen issues with ACPI tables above this waterline.
Simple - all we need to do then is enable 48-bit VA in the arm64 kernel config? Well, yes. I know Fedora are already doing this, and I have raised a bug[1] for Debian to do the same.
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=834505
The problem is - some pieces of software have had time to be written in a ... let's charitably call it a "focused on amd64" fasion ... with the embedded assumption that anything above virtual address bit 44 is a pointer-tag free-for-all.
On the Debian-ish side, we're coming up on both Ubuntu 16.10 and the freeze for Stretch, leaving a pretty short window to resolve this unholy kernel->initrd->userland triangle.
The applications we know are affected are luajit and mozjs (libv8 is not a problem). But this has a follow-on cost: both of these are used by other packages. Other jit/runtime packages could have their own issues.
The mozjs bug is fixed on trunk, and will hopefully make it into release 49[2], but it remains to be seen if that's too late for some distributions.
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1143022
For luajit, I'm told this has been fixed on 2.1 branch, but not merged to master?
Now, Jeremy (cc:d) tells me the list of currently-known Fedora packages affected by this are: couchdb elinks erlang-js freewrl libEAI libproxy-mozjs mediatomb pacrunner plowshare polkit cinnamon cjs cjs cjs-tests gjs gjs gjs-tests gnome-shell 0ad mongodb mongodb-server
Some of these may only need an updated luajit/mozjs package, but some may need more invasive changes.
Anyway, this is just a heads up - anyone sitting on more information than I've put into this email is very welcome to share it.
/ Leif