Okay, I went back through the kernel PPAs and in fact the most recent one that I can get to boot under 32-bit PV mode Xen is 4.13.16. 4.14-rc1 does not boot.
I realised then that it was possible that I had never tested any version of 32-bit Ubuntu 18.04 under Xen. It is possible that I only ever tested it 64-bit. The only reason why I am looking into this now is because a user with a 32-bit Ubuntu 16.04 did a release upgrade to 18.04.
On seeing that a large number of upstream kernel releases don't boot under 32-bit Xen, I did a quick check what the situation is in Debian. I know that I tested the current Debian stable both 32- and 64-bit. But that is based on kernel 4.9.x.
A 4.17.x kernel is available in stretch-backports, so I tried that, and sure enough that crashes too.
So, I'm a little shocked that I would be the first to notice this, but it seems like the upstream Linux kernel stopped working under 32-bit PV mode Xen quite some time ago.
I am more familiar with Debian so I am going to built the latest upstream stable kernel release on Debian and check that it also crashes in the same way, and if so then I guess I report it upstream, maybe do a bisection myself in Debian.
Okay, I went back through the kernel PPAs and in fact the most recent one that I can get to boot under 32-bit PV mode Xen is 4.13.16. 4.14-rc1 does not boot.
I realised then that it was possible that I had never tested any version of 32-bit Ubuntu 18.04 under Xen. It is possible that I only ever tested it 64-bit. The only reason why I am looking into this now is because a user with a 32-bit Ubuntu 16.04 did a release upgrade to 18.04.
On seeing that a large number of upstream kernel releases don't boot under 32-bit Xen, I did a quick check what the situation is in Debian. I know that I tested the current Debian stable both 32- and 64-bit. But that is based on kernel 4.9.x.
A 4.17.x kernel is available in stretch-backports, so I tried that, and sure enough that crashes too.
So, I'm a little shocked that I would be the first to notice this, but it seems like the upstream Linux kernel stopped working under 32-bit PV mode Xen quite some time ago.
I am more familiar with Debian so I am going to built the latest upstream stable kernel release on Debian and check that it also crashes in the same way, and if so then I guess I report it upstream, maybe do a bisection myself in Debian.