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

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Date:   Tue, 16 Jul 2019 05:55:20 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Stefano Stabellini <sstabellini@...nel.org>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        LinuxVirtualization <virtualization@...ts.linux-foundation.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Ingo Molnar <mingo@...hat.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 15.07.19 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@...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.)
>>
>> 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