Re: Moving from qemu-pr-helper and libmpathpersist to <linux/pr.h>

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Feb 05, 2026 at 12:52:33PM +0100, Martin Wilck wrote:
> On Wed, 2026-02-04 at 13:32 -0500, Stefan Hajnoczi wrote:
> > On Wed, Feb 04, 2026 at 02:19:48PM +0100, Martin Wilck wrote:
> > Getting back to the application vs DM-Multipath advantages: I think
> > it's
> > worth simplifying things for applications because there are many
> > applications and only one DM-Multipath.
> 
> TBH, I don't see so many applications. Actually I am having trouble
> finding any application at all that uses the generic linux PR
> functionality. I haven't even found a basic command line tool that
> encapsulates the ioctls, are you aware of one? That would be the first
> thing we need, be it only for testing the kernel.

blkpr(8) is part of util-linux:
https://github.com/util-linux/util-linux/blob/master/sys-utils/blkpr.c

> As for applications using SCSI/NVMe PRs, I also don't see many, at
> least not in the Linux / open source realm. Actually, qemu is the only
> one that immediately comes to my mind. I can imagine that storage
> management tools for e.g. OpenStack or Kubernetes would want to use
> PRs, but I don't know any details.

HA/clustering frameworks as well.

> Wrt sockets, not sure what's so painful about them. multipathd recently
> enabled a pathname sockets for qemu-pr-helper in KubeVirt [1].

This is a good example. KubeVirt has code to:
- Create a pr-helper container image with qemu-pr-helper,
  libmpathpersist, and a multipath.conf file.
- Run the pr-helper container with the host's multipathd socket and
  /etc/multipath directory passed through.
- Run a multipath-monitor daemon that remounts the multipathd UNIX
  domain socket bind mount when it changes (multipathd restart?).

And that's just at the KubeVirt level. Libvirt and QEMU also have code
to make this possible.

That is a lot of setup! All of this goes away if QEMU can use
<linux/pr.h> ioctls instead of libmpathpersist. It's just not needed.

Stefan

> 
> Thanks,
> Martin
> 
> [1] https://github.com/opensvc/multipath-tools/issues/111
> 

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