Re: [PATCH 0/2] NVMe namespace hotplug and drive reconnection support

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Apr 09, 2026 at 09:01:09AM +0200, mr-083 wrote:
> This series adds two features that together enable transparent NVMe disk
> hot-swap simulation in QEMU, matching the behavior of physical NVMe
> drives being pulled and reinserted in the same PCIe slot.
> 
> Problem:
> Currently, hot-swapping an NVMe disk in QEMU requires removing the
> entire NVMe controller via device_del, which causes the Linux guest to
> assign a new controller number on re-add (e.g. nvme2 becomes nvme4).
> This breaks storage software that tracks drives by device name.

Hi mr-083,
Neat, I was looking for something like this recently!

> Solution:
> Patch 1 adds hotplug support for nvme-ns devices on the NvmeBus, with
> proper Asynchronous Event Notification (AEN) so the guest kernel detects
> namespace changes. This allows namespace-level hot-swap without removing
> the NVMe controller.
> 
> Patch 2 adds a drive_insert HMP command that reconnects a host block
> device file to an existing guest device after drive_del. This is the
> counterpart to drive_del for non-removable devices where
> blockdev-change-medium cannot be used.
> 
> The recommended hot-swap sequence is:
>   1. drive_del <drive-id>          # disconnect backing store
>   2. drive_insert <device> <file>  # reconnect backing store

Is it possible to achieve this with device_del + device_add instead of
introducing a new monitor command?

device_del nvme-ns2
blockdev-del nvme-ns2-blk      (or drive_del)
...
blockdev-add nvme-ns2-blk,...  (or drive_add)
device_add nvme-ns,id=nvme-ns2,nsid=2,drive=nvme-ns2-blk

>   3. pcie_aer_inject_error <port> SDN  # trigger controller reset

Is NVMe AEN insufficient to get the guest to recognize the Namespace
change? I looked at the Linux NVMe driver code recently and got the
impression it would process changes to the Namespace list upon receiving
the NVMe AEN.

> After this sequence, the guest sees the same controller and namespace
> names (e.g. /dev/nvme2n1 remains /dev/nvme2n1), and the NVMe driver
> recovers transparently via the standard AER recovery path.
> 
> Tested with:
> - Linux 6.1 guest on QEMU aarch64 with HVF (macOS)
> - NVMe subsystem model with multipath disabled
> - DirectPV and MinIO AIStor storage stack
> 
> mr-083 (2):
>   hw/nvme: add namespace hotplug support
>   block/monitor: add drive_insert HMP command
> 
>  block/monitor/block-hmp-cmds.c | 59 +++++++++++++++++++++++
>  hmp-commands.hx                | 18 +++++++
>  hw/nvme/ctrl.c                 | 85 ++++++++++++++++++++++++++++++++++
>  hw/nvme/ns.c                   |  1 +
>  hw/nvme/subsys.c               |  2 +
>  include/block/block-hmp-cmds.h |  1 +
>  6 files changed, 166 insertions(+)
> 
> --
> 2.50.1 (Apple Git-155)
> 

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