[PATCH v3] hw/nvme: add namespace hotplug support

[PATCH v3] hw/nvme: add namespace hotplug support

[mr-083]

Add hotplug support for nvme-ns devices on the NvmeBus. This enables
NVMe namespace-level hot-add and hot-remove via device_add and
device_del with proper Asynchronous Event Notification (AEN), so the
guest kernel can react to namespace topology changes.

Mark nvme-ns devices as hotpluggable and register the NvmeBus as a
hotplug handler with proper plug and unplug callbacks:

- plug: attach namespace to all started controllers and send an
  Asynchronous Event Notification (AEN) with NS_ATTR_CHANGED so
  the guest kernel rescans namespaces and adds the block device
- unplug: drain in-flight I/O, detach from all controllers, send
  AEN, then unrealize the device. The guest kernel rescans and
  removes the block device.

The plug handler skips controllers that haven't started yet
(qs_created == false) to avoid interfering with boot-time namespace
attachment in nvme_start_ctrl().

The unplug handler drains in-flight I/O via nvme_ns_drain() before
detaching the namespace from controllers, so pending requests can
complete normally without touching freed state.

For symmetry with nvme_ns_realize() which sets subsys->namespaces[nsid],
nvme_ns_unrealize() now clears that slot too — making the namespace
lifecycle complete.

Both the controller bus and subsystem bus are configured as hotplug
handlers via qbus_set_bus_hotplug_handler() since nvme-ns devices
may reparent to the subsystem bus during realize.

Example hot-swap sequence using the NVMe subsystem model:

  # Boot with: -device nvme-subsys,id=subsys0
  #            -device nvme,id=ctrl0,subsys=subsys0
  #            -device nvme-ns,id=ns0,drive=drv0,bus=ctrl0,nsid=1

  device_del ns0             # guest receives AEN, removes /dev/nvme0n1
  drive_del drv0
  drive_add 0 file=disk.qcow2,format=qcow2,id=drv0,if=none
  device_add nvme-ns,id=ns0,drive=drv0,bus=ctrl0,nsid=1
                              # guest receives AEN, adds /dev/nvme0n1

Tested with Linux 6.1 guest (NVMe driver processes AEN and rescans
namespace list automatically).

Signed-off-by: Matthieu <matthieu@min.io>
---
 hw/nvme/ctrl.c   | 88 ++++++++++++++++++++++++++++++++++++++++++++++++
 hw/nvme/ns.c     |  8 +++++
 hw/nvme/subsys.c |  2 ++
 3 files changed, 98 insertions(+)

diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c
index be6c7028cb..2024b0ff75 100644
--- a/hw/nvme/ctrl.c
+++ b/hw/nvme/ctrl.c
@@ -206,6 +206,7 @@
 #include "system/hostmem.h"
 #include "hw/pci/msix.h"
 #include "hw/pci/pcie_sriov.h"
+#include "hw/core/qdev.h"
 #include "system/spdm-socket.h"
 #include "migration/vmstate.h"
 
@@ -9293,6 +9294,7 @@ static void nvme_realize(PCIDevice *pci_dev, Error **errp)
     }
 
     qbus_init(&n->bus, sizeof(NvmeBus), TYPE_NVME_BUS, dev, dev->id);
+    qbus_set_bus_hotplug_handler(BUS(&n->bus));
 
     if (nvme_init_subsys(n, errp)) {
         return;
@@ -9553,10 +9555,96 @@ static const TypeInfo nvme_info = {
     },
 };
 
+static void nvme_ns_hot_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
+                              Error **errp)
+{
+    NvmeNamespace *ns = NVME_NS(dev);
+    NvmeSubsystem *subsys = ns->subsys;
+    uint32_t nsid = ns->params.nsid;
+    int i;
+
+    /*
+     * Attach to all started controllers and notify via AEN.
+     * Skip controllers that haven't started yet (boot-time realize) —
+     * nvme_start_ctrl() will attach namespaces during controller init.
+     */
+    for (i = 0; i < NVME_MAX_CONTROLLERS; i++) {
+        NvmeCtrl *ctrl = nvme_subsys_ctrl(subsys, i);
+        if (!ctrl || !ctrl->qs_created) {
+            continue;
+        }
+
+        if (nvme_csi_supported(ctrl, ns->csi) && !ns->params.detached) {
+            nvme_attach_ns(ctrl, ns);
+            nvme_update_dsm_limits(ctrl, ns);
+
+            if (!test_and_set_bit(nsid, ctrl->changed_nsids)) {
+                nvme_enqueue_event(ctrl, NVME_AER_TYPE_NOTICE,
+                                   NVME_AER_INFO_NOTICE_NS_ATTR_CHANGED,
+                                   NVME_LOG_CHANGED_NSLIST);
+            }
+        }
+    }
+}
+
+static void nvme_ns_hot_unplug(HotplugHandler *hotplug_dev, DeviceState *dev,
+                               Error **errp)
+{
+    NvmeNamespace *ns = NVME_NS(dev);
+    NvmeSubsystem *subsys = ns->subsys;
+    uint32_t nsid = ns->params.nsid;
+    int i;
+
+    /*
+     * Drain in-flight I/O before tearing down the namespace.
+     * This must happen while the namespace is still attached to the
+     * controllers so any pending requests can complete normally.
+     */
+    nvme_ns_drain(ns);
+
+    /*
+     * Detach from all controllers and notify the guest via AEN.
+     * The guest kernel will rescan namespaces and remove the block device.
+     */
+    for (i = 0; i < NVME_MAX_CONTROLLERS; i++) {
+        NvmeCtrl *ctrl = nvme_subsys_ctrl(subsys, i);
+        if (!ctrl || !nvme_ns(ctrl, nsid)) {
+            continue;
+        }
+
+        nvme_detach_ns(ctrl, ns);
+        nvme_update_dsm_limits(ctrl, NULL);
+
+        if (!test_and_set_bit(nsid, ctrl->changed_nsids)) {
+            nvme_enqueue_event(ctrl, NVME_AER_TYPE_NOTICE,
+                               NVME_AER_INFO_NOTICE_NS_ATTR_CHANGED,
+                               NVME_LOG_CHANGED_NSLIST);
+        }
+    }
+
+    /*
+     * Unrealize: removes from subsystem (in nvme_ns_unrealize), flushes,
+     * cleans up structures, and removes from QOM.
+     */
+    qdev_unrealize(dev);
+}
+
+static void nvme_bus_class_init(ObjectClass *klass, const void *data)
+{
+    HotplugHandlerClass *hc = HOTPLUG_HANDLER_CLASS(klass);
+    hc->plug = nvme_ns_hot_plug;
+    hc->unplug = nvme_ns_hot_unplug;
+}
+
 static const TypeInfo nvme_bus_info = {
     .name = TYPE_NVME_BUS,
     .parent = TYPE_BUS,
     .instance_size = sizeof(NvmeBus),
+    .class_init = nvme_bus_class_init,
+    .interfaces = (const InterfaceInfo[]) {
+        { TYPE_HOTPLUG_HANDLER },
+        { }
+    },
 };
 
 static void nvme_register_types(void)
diff --git a/hw/nvme/ns.c b/hw/nvme/ns.c
index b0106eaa5c..f4f755c6fc 100644
--- a/hw/nvme/ns.c
+++ b/hw/nvme/ns.c
@@ -719,10 +719,17 @@ void nvme_ns_cleanup(NvmeNamespace *ns)
 static void nvme_ns_unrealize(DeviceState *dev)
 {
     NvmeNamespace *ns = NVME_NS(dev);
+    NvmeSubsystem *subsys = ns->subsys;
+    uint32_t nsid = ns->params.nsid;
 
     nvme_ns_drain(ns);
     nvme_ns_shutdown(ns);
     nvme_ns_cleanup(ns);
+
+    /* Symmetric with nvme_ns_realize() which sets subsys->namespaces[nsid]. */
+    if (subsys && nsid && subsys->namespaces[nsid] == ns) {
+        subsys->namespaces[nsid] = NULL;
+    }
 }
 
 void nvme_ns_atomic_configure_boundary(bool dn, uint16_t nabsn,
@@ -937,6 +944,7 @@ static void nvme_ns_class_init(ObjectClass *oc, const void *data)
     dc->bus_type = TYPE_NVME_BUS;
     dc->realize = nvme_ns_realize;
     dc->unrealize = nvme_ns_unrealize;
+    dc->hotpluggable = true;
     device_class_set_props(dc, nvme_ns_props);
     dc->desc = "Virtual NVMe namespace";
 }
diff --git a/hw/nvme/subsys.c b/hw/nvme/subsys.c
index 777e1c620f..fa35055d3c 100644
--- a/hw/nvme/subsys.c
+++ b/hw/nvme/subsys.c
@@ -9,6 +9,7 @@
 #include "qemu/osdep.h"
 #include "qemu/units.h"
 #include "qapi/error.h"
+#include "hw/core/qdev.h"
 
 #include "nvme.h"
 
@@ -205,6 +206,7 @@ static void nvme_subsys_realize(DeviceState *dev, Error **errp)
     NvmeSubsystem *subsys = NVME_SUBSYS(dev);
 
     qbus_init(&subsys->bus, sizeof(NvmeBus), TYPE_NVME_BUS, dev, dev->id);
+    qbus_set_bus_hotplug_handler(BUS(&subsys->bus));
 
     nvme_subsys_setup(subsys, errp);
 }
-- 
2.53.0

Re: [PATCH v3] hw/nvme: add namespace hotplug support

[Matthieu Rolla]

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Hello @Stefan Hajnoczi <stefanha@redhat.com> ,


Thanks for the review! v3 sent.


Wording: Fixed in v3, no more "physical PCIe slot" claim. Now describes it
as NVMe namespace-level hotplug.


I/O drain: Moved nvme_ns_drain() to the start of the unplug handler so
in-flight I/O completes before detach. Tested under warp load (16
concurrent 1MiB uploads via MinIO/DirectPV)  device_del returns in ~400ms
with clean removal, no use-after-free.


Symmetry:  moved subsys->namespaces[nsid] = NULL into nvme_ns_unrealize()
so the namespace lifecycle is complete (mirrors what nvme_ns_realize() sets
up).


I don't have a working Windows test setup, I'd really appreciate if you
could test it next week as you offered.

Thanks again for your time

On Fri, Apr 10, 2026 at 4:29 PM mr-083 <matthieu@minio.io> wrote:

> Add hotplug support for nvme-ns devices on the NvmeBus. This enables
> NVMe namespace-level hot-add and hot-remove via device_add and
> device_del with proper Asynchronous Event Notification (AEN), so the
> guest kernel can react to namespace topology changes.
>
> Mark nvme-ns devices as hotpluggable and register the NvmeBus as a
> hotplug handler with proper plug and unplug callbacks:
>
> - plug: attach namespace to all started controllers and send an
>   Asynchronous Event Notification (AEN) with NS_ATTR_CHANGED so
>   the guest kernel rescans namespaces and adds the block device
> - unplug: drain in-flight I/O, detach from all controllers, send
>   AEN, then unrealize the device. The guest kernel rescans and
>   removes the block device.
>
> The plug handler skips controllers that haven't started yet
> (qs_created == false) to avoid interfering with boot-time namespace
> attachment in nvme_start_ctrl().
>
> The unplug handler drains in-flight I/O via nvme_ns_drain() before
> detaching the namespace from controllers, so pending requests can
> complete normally without touching freed state.
>
> For symmetry with nvme_ns_realize() which sets subsys->namespaces[nsid],
> nvme_ns_unrealize() now clears that slot too — making the namespace
> lifecycle complete.
>
> Both the controller bus and subsystem bus are configured as hotplug
> handlers via qbus_set_bus_hotplug_handler() since nvme-ns devices
> may reparent to the subsystem bus during realize.
>
> Example hot-swap sequence using the NVMe subsystem model:
>
>   # Boot with: -device nvme-subsys,id=subsys0
>   #            -device nvme,id=ctrl0,subsys=subsys0
>   #            -device nvme-ns,id=ns0,drive=drv0,bus=ctrl0,nsid=1
>
>   device_del ns0             # guest receives AEN, removes /dev/nvme0n1
>   drive_del drv0
>   drive_add 0 file=disk.qcow2,format=qcow2,id=drv0,if=none
>   device_add nvme-ns,id=ns0,drive=drv0,bus=ctrl0,nsid=1
>                               # guest receives AEN, adds /dev/nvme0n1
>
> Tested with Linux 6.1 guest (NVMe driver processes AEN and rescans
> namespace list automatically).
>
> Signed-off-by: Matthieu <matthieu@min.io>
> ---
>  hw/nvme/ctrl.c   | 88 ++++++++++++++++++++++++++++++++++++++++++++++++
>  hw/nvme/ns.c     |  8 +++++
>  hw/nvme/subsys.c |  2 ++
>  3 files changed, 98 insertions(+)
>
> diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c
> index be6c7028cb..2024b0ff75 100644
> --- a/hw/nvme/ctrl.c
> +++ b/hw/nvme/ctrl.c
> @@ -206,6 +206,7 @@
>  #include "system/hostmem.h"
>  #include "hw/pci/msix.h"
>  #include "hw/pci/pcie_sriov.h"
> +#include "hw/core/qdev.h"
>  #include "system/spdm-socket.h"
>  #include "migration/vmstate.h"
>
> @@ -9293,6 +9294,7 @@ static void nvme_realize(PCIDevice *pci_dev, Error
> **errp)
>      }
>
>      qbus_init(&n->bus, sizeof(NvmeBus), TYPE_NVME_BUS, dev, dev->id);
> +    qbus_set_bus_hotplug_handler(BUS(&n->bus));
>
>      if (nvme_init_subsys(n, errp)) {
>          return;
> @@ -9553,10 +9555,96 @@ static const TypeInfo nvme_info = {
>      },
>  };
>
> +static void nvme_ns_hot_plug(HotplugHandler *hotplug_dev, DeviceState
> *dev,
> +                              Error **errp)
> +{
> +    NvmeNamespace *ns = NVME_NS(dev);
> +    NvmeSubsystem *subsys = ns->subsys;
> +    uint32_t nsid = ns->params.nsid;
> +    int i;
> +
> +    /*
> +     * Attach to all started controllers and notify via AEN.
> +     * Skip controllers that haven't started yet (boot-time realize) —
> +     * nvme_start_ctrl() will attach namespaces during controller init.
> +     */
> +    for (i = 0; i < NVME_MAX_CONTROLLERS; i++) {
> +        NvmeCtrl *ctrl = nvme_subsys_ctrl(subsys, i);
> +        if (!ctrl || !ctrl->qs_created) {
> +            continue;
> +        }
> +
> +        if (nvme_csi_supported(ctrl, ns->csi) && !ns->params.detached) {
> +            nvme_attach_ns(ctrl, ns);
> +            nvme_update_dsm_limits(ctrl, ns);
> +
> +            if (!test_and_set_bit(nsid, ctrl->changed_nsids)) {
> +                nvme_enqueue_event(ctrl, NVME_AER_TYPE_NOTICE,
> +                                   NVME_AER_INFO_NOTICE_NS_ATTR_CHANGED,
> +                                   NVME_LOG_CHANGED_NSLIST);
> +            }
> +        }
> +    }
> +}
> +
> +static void nvme_ns_hot_unplug(HotplugHandler *hotplug_dev, DeviceState
> *dev,
> +                               Error **errp)
> +{
> +    NvmeNamespace *ns = NVME_NS(dev);
> +    NvmeSubsystem *subsys = ns->subsys;
> +    uint32_t nsid = ns->params.nsid;
> +    int i;
> +
> +    /*
> +     * Drain in-flight I/O before tearing down the namespace.
> +     * This must happen while the namespace is still attached to the
> +     * controllers so any pending requests can complete normally.
> +     */
> +    nvme_ns_drain(ns);
> +
> +    /*
> +     * Detach from all controllers and notify the guest via AEN.
> +     * The guest kernel will rescan namespaces and remove the block
> device.
> +     */
> +    for (i = 0; i < NVME_MAX_CONTROLLERS; i++) {
> +        NvmeCtrl *ctrl = nvme_subsys_ctrl(subsys, i);
> +        if (!ctrl || !nvme_ns(ctrl, nsid)) {
> +            continue;
> +        }
> +
> +        nvme_detach_ns(ctrl, ns);
> +        nvme_update_dsm_limits(ctrl, NULL);
> +
> +        if (!test_and_set_bit(nsid, ctrl->changed_nsids)) {
> +            nvme_enqueue_event(ctrl, NVME_AER_TYPE_NOTICE,
> +                               NVME_AER_INFO_NOTICE_NS_ATTR_CHANGED,
> +                               NVME_LOG_CHANGED_NSLIST);
> +        }
> +    }
> +
> +    /*
> +     * Unrealize: removes from subsystem (in nvme_ns_unrealize), flushes,
> +     * cleans up structures, and removes from QOM.
> +     */
> +    qdev_unrealize(dev);
> +}
> +
> +static void nvme_bus_class_init(ObjectClass *klass, const void *data)
> +{
> +    HotplugHandlerClass *hc = HOTPLUG_HANDLER_CLASS(klass);
> +    hc->plug = nvme_ns_hot_plug;
> +    hc->unplug = nvme_ns_hot_unplug;
> +}
> +
>  static const TypeInfo nvme_bus_info = {
>      .name = TYPE_NVME_BUS,
>      .parent = TYPE_BUS,
>      .instance_size = sizeof(NvmeBus),
> +    .class_init = nvme_bus_class_init,
> +    .interfaces = (const InterfaceInfo[]) {
> +        { TYPE_HOTPLUG_HANDLER },
> +        { }
> +    },
>  };
>
>  static void nvme_register_types(void)
> diff --git a/hw/nvme/ns.c b/hw/nvme/ns.c
> index b0106eaa5c..f4f755c6fc 100644
> --- a/hw/nvme/ns.c
> +++ b/hw/nvme/ns.c
> @@ -719,10 +719,17 @@ void nvme_ns_cleanup(NvmeNamespace *ns)
>  static void nvme_ns_unrealize(DeviceState *dev)
>  {
>      NvmeNamespace *ns = NVME_NS(dev);
> +    NvmeSubsystem *subsys = ns->subsys;
> +    uint32_t nsid = ns->params.nsid;
>
>      nvme_ns_drain(ns);
>      nvme_ns_shutdown(ns);
>      nvme_ns_cleanup(ns);
> +
> +    /* Symmetric with nvme_ns_realize() which sets
> subsys->namespaces[nsid]. */
> +    if (subsys && nsid && subsys->namespaces[nsid] == ns) {
> +        subsys->namespaces[nsid] = NULL;
> +    }
>  }
>
>  void nvme_ns_atomic_configure_boundary(bool dn, uint16_t nabsn,
> @@ -937,6 +944,7 @@ static void nvme_ns_class_init(ObjectClass *oc, const
> void *data)
>      dc->bus_type = TYPE_NVME_BUS;
>      dc->realize = nvme_ns_realize;
>      dc->unrealize = nvme_ns_unrealize;
> +    dc->hotpluggable = true;
>      device_class_set_props(dc, nvme_ns_props);
>      dc->desc = "Virtual NVMe namespace";
>  }
> diff --git a/hw/nvme/subsys.c b/hw/nvme/subsys.c
> index 777e1c620f..fa35055d3c 100644
> --- a/hw/nvme/subsys.c
> +++ b/hw/nvme/subsys.c
> @@ -9,6 +9,7 @@
>  #include "qemu/osdep.h"
>  #include "qemu/units.h"
>  #include "qapi/error.h"
> +#include "hw/core/qdev.h"
>
>  #include "nvme.h"
>
> @@ -205,6 +206,7 @@ static void nvme_subsys_realize(DeviceState *dev,
> Error **errp)
>      NvmeSubsystem *subsys = NVME_SUBSYS(dev);
>
>      qbus_init(&subsys->bus, sizeof(NvmeBus), TYPE_NVME_BUS, dev, dev->id);
> +    qbus_set_bus_hotplug_handler(BUS(&subsys->bus));
>
>      nvme_subsys_setup(subsys, errp);
>  }
> --
> 2.53.0
>
>

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Re: [PATCH v3] hw/nvme: add namespace hotplug support

[Stefan Hajnoczi]

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On Fri, Apr 10, 2026 at 04:33:47PM +0200, Matthieu Rolla wrote:
> Hello @Stefan Hajnoczi <stefanha@redhat.com> ,
> 
> 
> Thanks for the review! v3 sent.
> 
> 
> Wording: Fixed in v3, no more "physical PCIe slot" claim. Now describes it
> as NVMe namespace-level hotplug.
> 
> 
> I/O drain: Moved nvme_ns_drain() to the start of the unplug handler so
> in-flight I/O completes before detach. Tested under warp load (16
> concurrent 1MiB uploads via MinIO/DirectPV)  device_del returns in ~400ms
> with clean removal, no use-after-free.
> 
> 
> Symmetry:  moved subsys->namespaces[nsid] = NULL into nvme_ns_unrealize()
> so the namespace lifecycle is complete (mirrors what nvme_ns_realize() sets
> up).
> 
> 
> I don't have a working Windows test setup, I'd really appreciate if you
> could test it next week as you offered.
> 
> Thanks again for your time

Awesome, thanks!

I will give Windows Server a spin next week. I'm not an expert in
hw/nvme/ but the patch looks good to me:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [PATCH v3] hw/nvme: add namespace hotplug support

[Matthieu Rolla]

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Hello,

I tested further more the device_del + device_add and the AEN works
correctly, causing the guest to rescan namespaces.

However, when XFS filesystems are mounted on the namespace (via
DirectPV/CSI in our case), the kernel doesn't reuse the block device number
after re-add.

For example, nvme2n1 becomes nvme2n2 because the stale XFS mount holds a
reference to the old nvme_ns_head, preventing ida_free() in
nvme_free_ns_head() from releasing the instance number before the new
namespace is allocated.

This causes XFS "duplicate UUID" errors since the same filesystem appears
under a different device name while the old stale mount still exists.

This is a Linux kernel behavior (ida_alloc in nvme_alloc_ns_head), not a
QEMU issue. The namespace hotplug patch itself is correct. But for
practical use with mounted filesystems, I found that the drive_del +
drive_insert approach (keeping the namespace device alive) avoids the
rename entirely since no ida allocation/free cycle occurs.

Would it make sense to include drive_insert as a companion patch, or is
that better handled separately ?

Thanks

On Fri, Apr 10, 2026 at 10:14 PM Stefan Hajnoczi <stefanha@redhat.com>
wrote:

> On Fri, Apr 10, 2026 at 04:33:47PM +0200, Matthieu Rolla wrote:
> > Hello @Stefan Hajnoczi <stefanha@redhat.com> ,
> >
> >
> > Thanks for the review! v3 sent.
> >
> >
> > Wording: Fixed in v3, no more "physical PCIe slot" claim. Now describes
> it
> > as NVMe namespace-level hotplug.
> >
> >
> > I/O drain: Moved nvme_ns_drain() to the start of the unplug handler so
> > in-flight I/O completes before detach. Tested under warp load (16
> > concurrent 1MiB uploads via MinIO/DirectPV)  device_del returns in ~400ms
> > with clean removal, no use-after-free.
> >
> >
> > Symmetry:  moved subsys->namespaces[nsid] = NULL into nvme_ns_unrealize()
> > so the namespace lifecycle is complete (mirrors what nvme_ns_realize()
> sets
> > up).
> >
> >
> > I don't have a working Windows test setup, I'd really appreciate if you
> > could test it next week as you offered.
> >
> > Thanks again for your time
>
> Awesome, thanks!
>
> I will give Windows Server a spin next week. I'm not an expert in
> hw/nvme/ but the patch looks good to me:
>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
>

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Re: [PATCH v3] hw/nvme: add namespace hotplug support

[Stefan Hajnoczi]

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On Mon, Apr 13, 2026 at 04:13:38PM +0200, Matthieu Rolla wrote:
> Hello Stefan,
> 
> Thanks let me know how it goes.

[Resending to qemu-devel so others are aware too.]

Hi Matthieu,
Windows Server 2022 Build 20348 successfully handles NVMe Namespace List
changes with your QEMU patch.

When adding the disk a new drive appears as expected.

When removing the disk the drive disappears and there is a Windows Event
Log event about a surprise removal. Windows probably has a procedure for
safely removing drives, but this basic test seems like enough
confirmation that it is working.

Drive letters are preserved across hotplug, so I think everything is
working as expected.

Stefan

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