Re: [Xen-devel] [PATCH 0/2] Remove 32-bit Xen PV guest support
From: Andrew Cooper <hidden>
Date: 2019-07-15 17:39:03
On 15/07/2019 18:28, Andy Lutomirski wrote:
On Mon, Jul 15, 2019 at 9:34 AM Andi Kleen [off-list ref] wrote:quoted
Juergen Gross [off-list ref] writes:quoted
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. Xen itself has been exclusively 64-bit since Xen 4.3 (released in 2013). Over time, various features like SMEP/SMAP have been making 32bit PV guests progressively slower, because ring 1 is supervisor rather than user. Things have got even worse with IBRS, to the point at which 32bit PV guests are starting to run like treacle. There are no current plans to remove support for 32bit PV guests from Xen, but it is very much in the category of "you shouldn't be using this mode any more". ~Andrew P.S. I don't see 64bit PV guest support going anywhere, because there are still a number of open performance questions due to the inherent differences between syscall and vmexit, and the difference EPT/NPT tables make on cross-domain mappings. _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization