Future of 32-bit PV support

Future of 32-bit PV support

[Juergen Gross]

In the Xen x86 community call we have been discussing whether anyone
really is depending on 32-bit PV guests. 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

- 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

And related to that:

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

- what about BSD? Is 32-bit PV support important there?


Juergen

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Re: Future of 32-bit PV support

[Christopher Clark]

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On Wed, Aug 15, 2018 at 11:17 PM, Juergen Gross <jgross@suse.com> wrote:

> In the Xen x86 community call we have been discussing whether anyone
> really is depending on 32-bit PV guests. 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
>
> - 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
>
> And related to that:
>
> - is there any Linux distribution still shipping 32-bit PV-capable
>   systems?
>
> - what about BSD? Is 32-bit PV support important there?
>


Juergen - just to be very clear about the scope here:
* would this proposal affect the ability to use a 32-bit dom0?
* is this proposal to affect x86 only or also for ARM?

Christopher

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Re: Future of 32-bit PV support

[Jan Beulich]

>>> On 16.08.18 at 08:17, <jgross@suse.com> wrote:
> In the Xen x86 community call we have been discussing whether anyone
> really is depending on 32-bit PV guests. 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

I think we shouldn't try to be too eager here. Deprecating is fine if
we can get sufficient confirmation that no-one needs this anymore,
but switching to something like the shim-only model needs to come
sufficiently much later.

For people concerned about memory footprint, should we perhaps
make x32 support in public headers and tool stack a prereq here?
Not the least because - afaict - 32-bit user space support in 64-bit
Linux continues to be subtly broken, due to there not being any
clear route for what to do with "x86-64/Xen: fix stack switching",
sent over three months ago? But even without that I think x32 is
preferable over true 32-bit mode.

Jan

> - 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
> 
> And related to that:
> 
> - is there any Linux distribution still shipping 32-bit PV-capable
>   systems?
> 
> - what about BSD? Is 32-bit PV support important there?
> 
> 
> Juergen
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xenproject.org 
> https://lists.xenproject.org/mailman/listinfo/xen-devel 





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Re: Future of 32-bit PV support

[Jan Beulich]

>>> On 16.08.18 at 08:32, <christopher.w.clark@gmail.com> wrote:
> On Wed, Aug 15, 2018 at 11:17 PM, Juergen Gross <jgross@suse.com> wrote:
> 
>> In the Xen x86 community call we have been discussing whether anyone
>> really is depending on 32-bit PV guests. 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
>>
>> - 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
>>
>> And related to that:
>>
>> - is there any Linux distribution still shipping 32-bit PV-capable
>>   systems?
>>
>> - what about BSD? Is 32-bit PV support important there?
>>
> 
> 
> Juergen - just to be very clear about the scope here:
> * would this proposal affect the ability to use a 32-bit dom0?

If the Dom0 is to be PV - yes, of course. For the time being there's
no complete PVH Dom0 support, so if 32-bit is needed here, PV is
for now indeed the only option.

> * is this proposal to affect x86 only or also for ARM?

There's no PV on ARM.

Jan



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Re: Future of 32-bit PV support

[Juergen Gross]

On 16/08/18 08:51, Jan Beulich wrote:
>>>> On 16.08.18 at 08:32, <christopher.w.clark@gmail.com> wrote:
>> On Wed, Aug 15, 2018 at 11:17 PM, Juergen Gross <jgross@suse.com> wrote:
>>
>>> In the Xen x86 community call we have been discussing whether anyone
>>> really is depending on 32-bit PV guests. 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
>>>
>>> - 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
>>>
>>> And related to that:
>>>
>>> - is there any Linux distribution still shipping 32-bit PV-capable
>>>   systems?
>>>
>>> - what about BSD? Is 32-bit PV support important there?
>>>
>>
>>
>> Juergen - just to be very clear about the scope here:
>> * would this proposal affect the ability to use a 32-bit dom0?
> 
> If the Dom0 is to be PV - yes, of course. For the time being there's
> no complete PVH Dom0 support, so if 32-bit is needed here, PV is
> for now indeed the only option.

And to be more precise: the first step would be to remove 32-bit PV
support from upstream Linux kernel. This would result in the loss of
the ability to use a _new_ Linux (e.g. >= 4.20 / 5.0) as a 32-bit dom0.
A 32-bit dom0 using a kernel <= 4.19 would still work until we remove
32-bit PV support from the hypervisor (which we wouldn't do before full
support of PVH dom0, I guess).

Is there a special reason you want to use a 32-bit dom0?


Juergen


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Re: Future of 32-bit PV support

[Roger Pau Monné]

On Thu, Aug 16, 2018 at 08:17:13AM +0200, Juergen Gross wrote:
> In the Xen x86 community call we have been discussing whether anyone
> really is depending on 32-bit PV guests. 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

This means 32bit PV support would be switched off by default in the
hypervisor build, but distros or individuals could still enable it
(like the build system will enable it for the shim).

> - 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
> 
> And related to that:
> 
> - is there any Linux distribution still shipping 32-bit PV-capable
>   systems?
> 
> - what about BSD? Is 32-bit PV support important there?

FTR, NetBSD is the only BSD to have PV support, both 32 and 64bits.

Roger.

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Re: Future of 32-bit PV support

[Stefano Stabellini]

On Thu, 16 Aug 2018, Juergen Gross wrote:
> In the Xen x86 community call we have been discussing whether anyone
> really is depending on 32-bit PV guests. 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
> 
> - 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
> 
> And related to that:
> 
> - is there any Linux distribution still shipping 32-bit PV-capable
>   systems?
> 
> - what about BSD? Is 32-bit PV support important there?

Hi Juergen,

Although I can see that deprecating 32-bit PV guest support is
desirable, and it might not cause any problems to Linux and
BSDs, we need to be careful about unikernels.

There are probably unikernels out there that only support PV 32bit
still. And why not? If you are designing a unikernel today, it would
still make sense to use PV 32bit or PVH.

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Re: Future of 32-bit PV support

[Christopher Clark]

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On Thu, Aug 16, 2018 at 12:55 AM, Juergen Gross <jgross@suse.com> wrote:

> On 16/08/18 08:51, Jan Beulich wrote:
> >>>> On 16.08.18 at 08:32, <christopher.w.clark@gmail.com> wrote:
> >> On Wed, Aug 15, 2018 at 11:17 PM, Juergen Gross <jgross@suse.com>
> wrote:
> >>
> >>> In the Xen x86 community call we have been discussing whether anyone
> >>> really is depending on 32-bit PV guests. 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
> >>>
> >>> - 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
> >>>
> >>> And related to that:
> >>>
> >>> - is there any Linux distribution still shipping 32-bit PV-capable
> >>>   systems?
> >>>
> >>> - what about BSD? Is 32-bit PV support important there?
> >>
> >> Juergen - just to be very clear about the scope here:
> >> * would this proposal affect the ability to use a 32-bit dom0?
> >
> > If the Dom0 is to be PV - yes, of course. For the time being there's
> > no complete PVH Dom0 support, so if 32-bit is needed here, PV is
> > for now indeed the only option.
>

ack. I asked because it's not necessarily obvious to all that dom0 is
included in the term "guest", and it widens the consequences of this change.


>
> And to be more precise: the first step would be to remove 32-bit PV
> support from upstream Linux kernel. This would result in the loss of
> the ability to use a _new_ Linux (e.g. >= 4.20 / 5.0) as a 32-bit dom0.
> A 32-bit dom0 using a kernel <= 4.19 would still work until we remove
> 32-bit PV support from the hypervisor (which we wouldn't do before full
> support of PVH dom0, I guess).
>

That makes sense.


>
> Is there a special reason you want to use a 32-bit dom0?
>

In short, no.
OpenXT currently uses a 32-bit PV dom0, but work is already under way to
migrate to 64-bit. I think your proposal is justified and good.

Christopher

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

[Andy Smith]

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

[Juergen Gross]

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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Re: Future of 32-bit PV support

[Juergen Gross]

On 16/08/18 19:34, Stefano Stabellini wrote:
> On Thu, 16 Aug 2018, Juergen Gross wrote:
>> In the Xen x86 community call we have been discussing whether anyone
>> really is depending on 32-bit PV guests. 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
>>
>> - 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
>>
>> And related to that:
>>
>> - is there any Linux distribution still shipping 32-bit PV-capable
>>   systems?
>>
>> - what about BSD? Is 32-bit PV support important there?
> 
> Hi Juergen,
> 
> Although I can see that deprecating 32-bit PV guest support is
> desirable, and it might not cause any problems to Linux and
> BSDs, we need to be careful about unikernels.
> 
> There are probably unikernels out there that only support PV 32bit
> still. And why not? If you are designing a unikernel today, it would
> still make sense to use PV 32bit or PVH.

PVH will still work, of course. 32- and 64-bit.


Juergen

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Re: Future of 32-bit PV support

[Manuel Bouyer]

On Thu, Aug 16, 2018 at 08:17:13AM +0200, Juergen Gross wrote:
> In the Xen x86 community call we have been discussing whether anyone
> really is depending on 32-bit PV guests. 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
> 
> - 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
> 
> And related to that:
> 
> - is there any Linux distribution still shipping 32-bit PV-capable
>   systems?
> 
> - what about BSD? Is 32-bit PV support important there?

Hello,
Actually, NetBSD only supports PV (there is work toward PV drivers for HVM,
and PVH, but this is making slow progress). 32-bit PV is faster than 64-bit PV
so all my domUs are 32bits these days.

-- 
Manuel Bouyer <bouyer@antioche.eu.org>
     NetBSD: 26 ans d'experience feront toujours la difference
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Re: Future of 32-bit PV support

[Juergen Gross]

On 20/08/18 17:31, Manuel Bouyer wrote:
> On Thu, Aug 16, 2018 at 08:17:13AM +0200, Juergen Gross wrote:
>> In the Xen x86 community call we have been discussing whether anyone
>> really is depending on 32-bit PV guests. 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
>>
>> - 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
>>
>> And related to that:
>>
>> - is there any Linux distribution still shipping 32-bit PV-capable
>>   systems?
>>
>> - what about BSD? Is 32-bit PV support important there?
> 
> Hello,
> Actually, NetBSD only supports PV (there is work toward PV drivers for HVM,
> and PVH, but this is making slow progress). 32-bit PV is faster than 64-bit PV
> so all my domUs are 32bits these days.

Thanks for the feedback.


Juergen


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

[Hans van Kranenburg]

On 08/17/2018 06:09 AM, Juergen Gross wrote:
> On 17/08/18 00:33, Andy Smith wrote:
>> Hi Juergen,
>>
>> [...] 
>> 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.

+1, Yes please!

Do I understand correctly ('another kernel enhancement') that you found
more things which need to be done than the 32-bit memory map limitation
issue at the grub side?

>> [...]
Thanks,
Hans

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

[Juergen Gross]

On 20/08/18 18:01, Hans van Kranenburg wrote:
> On 08/17/2018 06:09 AM, Juergen Gross wrote:
>> On 17/08/18 00:33, Andy Smith wrote:
>>> Hi Juergen,
>>>
>>> [...] 
>>> 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.
> 
> +1, Yes please!
> 
> Do I understand correctly ('another kernel enhancement') that you found
> more things which need to be done than the 32-bit memory map limitation
> issue at the grub side?

The kernel is still lacking support for handing over RSDP address from
grub to the kernel. The initial patches adding that haven't been taken
as several distributions are shipping a patched grub2 which is breaking
the grub/kernel boot protocol by clobbering kernel data.

I need to extend that protocol to detect such a clobbering in order to
be able to tell the difference between grub having set a new parameter
or just written garbage to it.


Juergen

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