Re: [PATCH 0/2] Remove 32-bit Xen PV guest support

From: Juergen Gross
Date: Mon Jul 15 2019 - 13:42:39 EST


On 15.07.19 19:28, Andy Lutomirski wrote:
On Mon, Jul 15, 2019 at 9:34 AM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:

Juergen Gross <jgross@xxxxxxxx> writes:

The long term plan has been to replace Xen PV guests by PVH. The first
victim of that plan are now 32-bit PV guests, as those are used only
rather seldom these days. Xen on x86 requires 64-bit support and with
Grub2 now supporting PVH officially since version 2.04 there is no
need to keep 32-bit PV guest support alive in the Linux kernel.
Additionally Meltdown mitigation is not available in the kernel running
as 32-bit PV guest, so dropping this mode makes sense from security
point of view, too.

Normally we have a deprecation period for feature removals like this.
You would make the kernel print a warning for some releases, and when
no user complains you can then remove. If a user complains you can't.


As I understand it, the kernel rules do allow changes like this even
if there's a complaint: this is a patch that removes what is
effectively hardware support. If the maintenance cost exceeds the
value, then removal is fair game. (Obviously we weight the value to
preserving compatibility quite highly, but in this case, Xen dropped
32-bit hardware support a long time ago. If the Xen hypervisor says
that 32-bit PV guest support is deprecated, it's deprecated.)

That being said, a warning might not be a bad idea. What's the
current status of this in upstream Xen?

Xen still supports that.

We have asked downstream for their opinion about dropping 32-bit PV
guest support in the kernel about 1 year ago and the common answer was:
no problem, but for users still wanting 32 bit guests we should wait
until PVH support is available in all related products. Grub2 was the
last one missing and as grub2 has released a version with PVH support
I posted this small series now.


Juergen