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

[Juergen Gross <jgross@xxxxxxxx>]

On 15.07.19 19:39, Andrew Cooper wrote:
> On 15/07/2019 18:28, 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?
> So personally, I'd prefer to see support stay, but at the end of the day
> it is Juergen's choice as the maintainer of the code.
Especially on the security front we are unsafe with 32-bit PV Linux.
And making it safe will make it so slow that the needed effort is not
spent very well.


Juergen

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