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

[Juergen Gross <jgross@xxxxxxxx>]

On 17/08/18 00:33, Andy Smith wrote:
> Hi Juergen,
> 
> As this was also addressed to -user I'm going to assume that you do
> want user response as well.

Right. Thanks for responding.

> 
> 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.

Is something like missing Meltdown mitigation for 32-bit PV guest a
technical reason?

> 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.

Okay, noted.

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

Yes.

> 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?

I think this is a reasonable request.

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

I have a patch series lying around for grub2 PVH support. It requires
some rework and another kernel enhancement. I'll try to resume work on
the patches soon.

> 
>> - 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 guess they will stick to the stable kernel they are using now? Then
this will be no problem.

> 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.

In the end it should be just a switch of domain type and boot loader
(PV -> PVH, grubxen -> grubxen-pvh). The kernel needs to be configured
to support PVH, of course.

Thanks for the very valuable input!


Juergen

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