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

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Date:   Tue, 16 Jul 2019 06:58:14 +0000
From:   Jan Beulich <JBeulich@...e.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Andi Kleen <ak@...ux.intel.com>, Borislav Petkov <bp@...en8.de>,
        "Stefano Stabellini" <sstabellini@...nel.org>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juergen Gross <JGross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alok Kataria <akataria@...are.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 0/2] Remove 32-bit Xen PV guest support

On 15.07.2019 19:28, Andy Lutomirski wrote:
> On Mon, Jul 15, 2019 at 9:34 AM Andi Kleen <ak@...ux.intel.com> wrote:
>>
>> Juergen Gross <jgross@...e.com> 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.)

Since it was implied but not explicit from Andrew's reply, just to
make it explicit: So far 32-bit PV guest support has not been
deprecated in Xen itself.

Jan