Re: [Xen-devel] [Xen-users] Future of 32-bit PV support

[Andy Smith <andy@xxxxxxxxxxxxxx>]

Hi Juergen,

As this was also addressed to -user I'm going to assume that you do
want user response as well.

On Thu, Aug 16, 2018 at 08:17:13AM +0200, Juergen Gross wrote:
> We'd like to evaluate whether anyone would see problems with:
> 
> - deprecating 32-bit PV guest support in Xen, meaning that we'd
>   eventually switch to support 32-bit PV guests only via PV-shim from
>   Xen 4.12 or 4.13

Although amd64 has been the default for us for many years, at the
moment we still have 64% of our customers running 32-bit PV. If
there remains a way for us to boot them through PV-shim and then
pvgrub2 with no functional changes and no work inside the guest then
that's fine, we'll adapt.

> - dropping 32-bit PV support from upstream Linux kernel, resulting in
>   current 32-bit PV guests no longer being able to upgrade to the newest
>   kernel version any longer

I doubt there is any technical reason why they can't switch to
64-bit, it's just that in the majority of cases that involves a
complete reinstall and the users just haven't bothered to.

If they are forced to switch because an impending kernel update will
leave them with a kernel that doesn't boot, they are going to be
upset that they are forced to reinstall their guest, or switch to a
64-bit kernel with their existing 32-bit userland.

It will of course help if they have plenty of warning that they need
to make the switch. But unless we're talking 2+ years of warning I'm
sure there will be some who will be unhappy.

I was hoping to transition to PVH guests as soon as possible, but
last time I looked into it there was a problem booting the stable
Linux kernel under PVH, and also no support in grub2.

Will it remain possible to boot a 32-bit Linux guest in PVH mode?

If so, could the final removal of 32-bit PV in the Linux kernel be
held off until there is:

1) a kernel shipping in Debian stable, Ubuntu LTS and CentOS that
   boots under PVH, and;

2) support in grub2 so I can build a grub image that boots under
   PVH?

If grub PVH support is not going to happen, what is the roadmap for
user-specified guest kernels under PVH?

> - is there any Linux distribution still shipping 32-bit PV-capable
>   systems?

Debian stable 32-bit kernels still boot under PV, as do Ubuntu 18.04
LTS ones. Ubuntu LTS releases are supposed to be supported (by
Canonical) for 5 years, and while of course Xen does not fall under
the category of software that they support, there will be people
sticking with 18.04 LTS as long as they can.

I'm not saying that people running 32-bit PV Ubuntu 18.04 are right
to expect that to continue being supported until 2023. I'm just
saying that human nature dictates that those sorts of expectations
will exist.

It will help a lot if there is an easy way for us to switch them
from 32-bit PV to PVH, while still letting them install their own
kernels.

Cheers,
Andy

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