Bug 2158705

Summary:	RFE: Prefer /dev/userfaultfd over userfaultfd(2) syscall
Product:	Red Hat Enterprise Linux 9	Reporter:	Michal Privoznik <mprivozn>
Component:	libvirt	Assignee:	Jiri Denemark <jdenemar>
libvirt sub component:	Live Migration	QA Contact:	Fangge Jin <fjin>
Status:	CLOSED MIGRATED	Docs Contact:
Severity:	unspecified
Priority:	unspecified	CC:	jsuchane, lcheng, lmen, virt-bugs, virt-maint
Version:	9.2	Keywords:	FutureFeature, MigratedToJIRA, Triaged
Target Milestone:	rc	Flags:	pm-rhel: mirror+
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	Enhancement
Doc Text:		Story Points:	---
Clone Of:	2158704	Environment:
Last Closed:	2023-09-22 13:14:37 UTC	Type:	Story
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	2158704
Bug Blocks:

Description Michal Privoznik 2023-01-06 08:25:13 UTC

+++ This bug was initially created as a clone of Bug #2158704 +++

Description of problem:
So far, for postcopy migration the userfaultfd(2) syscall is used. But this has couple of drawbacks (which are summarized nicely in kernel commit [1]). To resolve these, kernel came up with /dev/userfaultfd device, and this is a request to switch to that.

Please note, some scenarios where QEMU is running might be disallowing the userfaultfd(2) syscall as it is viewed as too powerful. For intsance KubeVirt [2].


1: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2d5de004e009add27db76c5cdc9f1f7f7dc087e7

2: https://issues.redhat.com/browse/OCPBUGS-5031

Comment 1 Michal Privoznik 2023-01-06 14:34:08 UTC

We still need to discuss the design of the feature, but one of the ideas was that libvirtd would open the file and pass FD to QEMU.

Comment 2 John Ferlan 2023-01-09 20:32:20 UTC

Also see the "work-around" of sorts in the plan for kubevirt (https://github.com/kubevirt/kubevirt/pull/8917).

Comment 3 RHEL Program Management 2023-09-22 13:12:15 UTC

Issue migration from Bugzilla to Jira is in process at this time. This will be the last message in Jira copied from the Bugzilla bug.

Comment 4 RHEL Program Management 2023-09-22 13:14:37 UTC

This BZ has been automatically migrated to the issues.redhat.com Red Hat Issue Tracker. All future work related to this report will be managed there.

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