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Re: [PATCH 4/4] hw/nvme: fix controller hot unplugging

From: Klaus Jensen
Subject: Re: [PATCH 4/4] hw/nvme: fix controller hot unplugging
Date: Wed, 7 Jul 2021 17:41:15 +0200

On Jul  7 12:43, Hannes Reinecke wrote:
On 7/7/21 11:53 AM, Klaus Jensen wrote:
On Jul  7 09:49, Hannes Reinecke wrote:
On 7/6/21 11:33 AM, Klaus Jensen wrote:
From: Klaus Jensen <k.jensen@samsung.com>

Prior to this patch the nvme-ns devices are always children of the
NvmeBus owned by the NvmeCtrl. This causes the namespaces to be
unrealized when the parent device is removed. However, when subsystems
are involved, this is not what we want since the namespaces may be
attached to other controllers as well.

This patch adds an additional NvmeBus on the subsystem device. When
nvme-ns devices are realized, if the parent controller device is linked
to a subsystem, the parent bus is set to the subsystem one instead. This
makes sure that namespaces are kept alive and not unrealized.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
 hw/nvme/nvme.h   | 18 ++++++++++--------
 hw/nvme/ctrl.c   |  8 +++++---
 hw/nvme/ns.c     | 32 +++++++++++++++++++++++++-------
 hw/nvme/subsys.c |  4 ++++
 4 files changed, 44 insertions(+), 18 deletions(-)

diff --git a/hw/nvme/nvme.h b/hw/nvme/nvme.h
index c4065467d877..9401e212f9f7 100644
--- a/hw/nvme/nvme.h
+++ b/hw/nvme/nvme.h
 typedef struct NvmeCtrl NvmeCtrl;
 typedef struct NvmeNamespace NvmeNamespace;
+#define TYPE_NVME_BUS "nvme-bus"
+typedef struct NvmeBus {
+    BusState parent_bus;
+    bool     is_subsys;
+} NvmeBus;
 #define TYPE_NVME_SUBSYS "nvme-subsys"
 #define NVME_SUBSYS(obj) \
     OBJECT_CHECK(NvmeSubsystem, (obj), TYPE_NVME_SUBSYS)
 typedef struct NvmeSubsystem {
     DeviceState parent_obj;
+    NvmeBus     bus;
     uint8_t     subnqn[256];
     NvmeCtrl      *ctrls[NVME_MAX_CONTROLLERS];
@@ -365,13 +374,6 @@ typedef struct NvmeCQueue {
     QTAILQ_HEAD(, NvmeRequest) req_list;
 } NvmeCQueue;
-#define TYPE_NVME_BUS "nvme-bus"
-#define NVME_BUS(obj) OBJECT_CHECK(NvmeBus, (obj), TYPE_NVME_BUS)
-typedef struct NvmeBus {
-    BusState parent_bus;
-} NvmeBus;
 #define TYPE_NVME "nvme"
 #define NVME(obj) \
         OBJECT_CHECK(NvmeCtrl, (obj), TYPE_NVME)
@@ -463,7 +465,7 @@ typedef struct NvmeCtrl {
 static inline NvmeNamespace *nvme_ns(NvmeCtrl *n, uint32_t nsid)
-    if (!nsid || nsid > NVME_MAX_NAMESPACES) {
+    if (!n || !nsid || nsid > NVME_MAX_NAMESPACES) {
         return NULL;
diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c
index 90e3ee2b70ee..7c8fca36d9a5 100644
--- a/hw/nvme/ctrl.c
+++ b/hw/nvme/ctrl.c
@@ -6516,11 +6516,13 @@ static void nvme_exit(PCIDevice *pci_dev)
     for (i = 1; i <= NVME_MAX_NAMESPACES; i++) {
         ns = nvme_ns(n, i);
-        if (!ns) {
-            continue;
+        if (ns) {
+            ns->attached--;
+    }
-        nvme_ns_cleanup(ns);
+    if (n->subsys) {
+        nvme_subsys_unregister_ctrl(n->subsys, n);
     if (n->subsys) {
diff --git a/hw/nvme/ns.c b/hw/nvme/ns.c
index 3c4f5b8c714a..612a2786d75d 100644
--- a/hw/nvme/ns.c
+++ b/hw/nvme/ns.c
@@ -444,13 +444,29 @@ void nvme_ns_cleanup(NvmeNamespace *ns)
 static void nvme_ns_realize(DeviceState *dev, Error **errp)
     NvmeNamespace *ns = NVME_NS(dev);
-    BusState *s = qdev_get_parent_bus(dev);
-    NvmeCtrl *n = NVME(s->parent);
-    NvmeSubsystem *subsys = n->subsys;
+    BusState *qbus = qdev_get_parent_bus(dev);
+    NvmeBus *bus = NVME_BUS(qbus);
+    NvmeCtrl *n = NULL;
+    NvmeSubsystem *subsys = NULL;
     uint32_t nsid = ns->params.nsid;
     int i;
-    if (!n->subsys) {
+    if (bus->is_subsys) {
+        subsys = NVME_SUBSYS(qbus->parent);
+    } else {
+        n = NVME(qbus->parent);
+        subsys = n->subsys;
+    }
+    if (subsys) {
+        /*
+         * If this namespace belongs to a subsystem (through a link on the
+         * controller device), reparent the device.
+         */
+        if (!qdev_set_parent_bus(dev, &subsys->bus.parent_bus, errp)) {
+            return;
+        }
+    } else {
         if (ns->params.detached) {
             error_setg(errp, "detached requires that the nvme device is "
                        "linked to an nvme-subsys device");
@@ -470,7 +486,7 @@ static void nvme_ns_realize(DeviceState *dev, Error **errp)
     if (!nsid) {
         for (i = 1; i <= NVME_MAX_NAMESPACES; i++) {
-            if (nvme_ns(n, i) || nvme_subsys_ns(subsys, i)) {
+            if (nvme_subsys_ns(subsys, i) || nvme_ns(n, i)) {
@@ -483,7 +499,7 @@ static void nvme_ns_realize(DeviceState *dev, Error **errp)
     } else {
-        if (nvme_ns(n, nsid) || nvme_subsys_ns(subsys, nsid)) {
+        if (nvme_subsys_ns(subsys, nsid) || nvme_ns(n, nsid)) {
             error_setg(errp, "namespace id '%d' already allocated", nsid);
@@ -509,7 +525,9 @@ static void nvme_ns_realize(DeviceState *dev, Error **errp)
-    nvme_attach_ns(n, ns);
+    if (n) {
+        nvme_attach_ns(n, ns);
+    }
 static Property nvme_ns_props[] = {
diff --git a/hw/nvme/subsys.c b/hw/nvme/subsys.c
index 92caa604a280..fb7c3a7c55fc 100644
--- a/hw/nvme/subsys.c
+++ b/hw/nvme/subsys.c
@@ -50,6 +50,10 @@ static void nvme_subsys_realize(DeviceState *dev, Error **errp)
     NvmeSubsystem *subsys = NVME_SUBSYS(dev);
+    qbus_create_inplace(&subsys->bus, sizeof(NvmeBus), TYPE_NVME_BUS, dev,
+                        dev->id);
+    subsys->bus.is_subsys = true;

Why not always create a subsystem, and distinguish between 'shared' and 'non-shared' subsystems instead of the ->subsys pointer? That way all namespaces can be children of the subsystem, we won't need any reparenting, and the whole thing will be more in-line with qdev, no?

Hi Hannes,

Thanks for your reviews and comments!

So, I have actually considered what you suggest.

The problem is that the nvme-ns device currently expects to plug into a bus (an 'nvme-bus') and we cannot really get rid of that 'bus' parameter without breaking compatibility. I experimented with removing the bus from the nvme device and instead creating it in the nvme-subsys device. My idea here was to implicitly always create an nvme-subsys if not explicitly created by the user, but since nvme-subsys does not plug into any bus itself, it is not exposed in the qtree and thus not reachable (i.e., when realizing the nvme-ns device, it will complain that there are no 'nvme-bus' available to plug into). To make this work, I believe we'd have to have the nvme-subsys plug into the main system bus (or some other bus rooted at main-system-bus), and I'd prefer not to do that since this is already a flawed design and I think it would be more intrusive than what my patch does.


I seem to trip over issues for which no patch can possibly be accepted as first the infrastructure has to be reworked.

... there is a reason why I'm avoiding posting patches to qemu-devel :-(

Actually, I'm the one to blame for making nvme-ns a -device and partly for why nvme-subsys is one as well... It's made sense at the time and thats why the infrastructure is biting me in the behind now ;)

I raised these issues on the mailinglist some time ago[1]. We didn't really find a good solution, but I think the conclusion is that the bus/device design of subsystems and namespaces is flawed. That's why I am working on an "objectification" of the two devices as suggested by Stefan[2] as a proper fix for this mess.

Actually, I _do_ think that it would be good to have an nvme-subsystem bus, providinga list of all namespaces for that subsystem.

My goal was to *not* use a qbus I don't think the subsystem should be modelled as a device. The same can be accomplished with -objects and links.

Creating a controller would then mean that one has to create a controller and a namespace _separately_, as then the namespace would _not_ be a child of the controller. But that's arguably a good thing, as the namespaces _do_ have a separate lifetime from controllers. And the lifetime of the subsystem could be completely self-contained; the controller would be looking up subsystems via the subsystem NQN if present; I guess we'll need to create ad-hoc subsystems for PCIe.
But nothing insurmountable.
And certainly nothing which require a large-scale rework of qemu internals, I would think.

Yes, exactly, totally agree.

But _if_ you work on the rework, please ensure that not only NVMe is represented here. SCSI multipathing has the same issue currently; there is a strict host->lun->block->OS device relationship currently in SCSI, making it impossible to represent a multipathed SCSI device in qemu. The only sneaky way is to allow for a shared OS device (ie specifying the filename more than once), but that completely sidesteps qemu internals, too.

I'm only very superficially familiar with hw/scsi, but this sounds more like the scsi subsystem also has some similar design problems like hw/nvme, not an inherent issue with QDEV/QOM.

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