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[Qemu-devel] [RFC PATCH v2 1/1] s390x/css: unrestrict cssids

From: Halil Pasic
Subject: [Qemu-devel] [RFC PATCH v2 1/1] s390x/css: unrestrict cssids
Date: Tue, 28 Nov 2017 14:07:58 +0100

The default css 0xfe is currently restricted to virtual subchannel
devices. The hope when the decision was made was, that non-virtual
subchannel devices will come around when guest can exploit multiple
channel subsystems. Since the guests generally don't do, the pain
of the partitioned (cssid) namespace outweighs the gain.

Let us remove the corresponding restrictions (virtual devices
can be put only in 0xfe and non-virtual devices in any css except
the 0xfe -- while s390-squash-mcss then remaps everything to cssid 0).

With this change, however, our schema for generating a css bus ids, if
none is specified does not make much sense. Currently we start at cssid
0x0 for non-virtual devices and use the default css (without
s390-squash-mcss exclusively) for virtual devices.  That means for
non-virtual device the device would auto-magically end up non-visible for
guests (which can't use the other channel subsystems).

Thus let us also change the css bus id auto assignment algorithm,
so that we first fill the default css, and then proceed to the
next one (modulo MAX_CSSID).

The adverse effect of getting rid of the restriction on migration should
not be too severe.  Vfio-ccw devices are not live-migratable yet, and for
virtual devices using the extra freedom would only make sense with the
aforementioned guest support in place.

The auto-generated bus ids are affected by both changes. We hope to not
encounter any auto-generated bus ids in production as Libvirt is always
explicit about the bus id.  Since 8ed179c937 ("s390x/css: catch section
mismatch on load", 2017-05-18) the worst that can happen because the same
device ended up having a different bus id is a cleanly failed migration.
I find it hard to reason about the impact of changed auto-generated bus
ids on migration for command line users as I don't know which rules is
such an user supposed to follow.

Another pain-point is down- or upgrade of QEMU for command line users.
The old way and the new way of doing vfio-ccw are mutually incompatible.
Libvirt is only going to support the new way, so for libvirt users, the
possible problems at QEMU downgrade are the following. If a domain
contains virtual devices placed into a css different than 0xfe the domain
wil refuse to start with a QEMU not having this patch. Putting devices
into a css different that 0xfe however won't make much sense in the
near future (guest support). Libvirt will refuse to do vfio-ccw with
a QEMU not having this patch. This is business as usual.

Signed-off-by: Halil Pasic <address@hidden>

Hi all!

Moving the property to the machine turned out very ugly (IMHO).  Libvirt
detects machine properties via query-command-line-options.  This is
however decoupled from the existence of the actual property: one needs to
touch util/qemu-config.c (see patch) so the property shows up.
Furthermore this stuff is global. I've also noticed that the infamous
s390-squash-mcss does not show up.

On the other hand to get the property displayed by -machine
s390-ccw-virtio,help one needs a setter on the property. So I have
created a fake setter which produces an error each time called.

I would strongly prefer putting back the property to the device level!

v1 -> v2:
* changed ccw bus id generation too (see commit message)
* moved the property to the machine (see cover letter)
* added a description to the property
 hw/s390x/3270-ccw.c        |  2 +-
 hw/s390x/css.c             | 28 ++++------------------------
 hw/s390x/s390-ccw.c        |  2 +-
 hw/s390x/s390-virtio-ccw.c | 21 +++++++++++++++++++++
 hw/s390x/virtio-ccw.c      |  2 +-
 include/hw/s390x/css.h     | 12 ++++--------
 util/qemu-config.c         |  6 ++++++
 7 files changed, 38 insertions(+), 35 deletions(-)

diff --git a/hw/s390x/3270-ccw.c b/hw/s390x/3270-ccw.c
index 081e3ef6f4..3af13ea027 100644
--- a/hw/s390x/3270-ccw.c
+++ b/hw/s390x/3270-ccw.c
@@ -104,7 +104,7 @@ static void emulated_ccw_3270_realize(DeviceState *ds, 
Error **errp)
     SubchDev *sch;
     Error *err = NULL;
-    sch = css_create_sch(cdev->devno, true, cbus->squash_mcss, errp);
+    sch = css_create_sch(cdev->devno, cbus->squash_mcss, errp);
     if (!sch) {
diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index f6b5c807cd..cd26f32050 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -2370,22 +2370,12 @@ const PropertyInfo css_devid_ro_propinfo = {
     .get = get_css_devid,
-SubchDev *css_create_sch(CssDevId bus_id, bool is_virtual, bool squash_mcss,
-                         Error **errp)
+SubchDev *css_create_sch(CssDevId bus_id, bool squash_mcss, Error **errp)
     uint16_t schid = 0;
     SubchDev *sch;
     if (bus_id.valid) {
-        if (is_virtual != (bus_id.cssid == VIRTUAL_CSSID)) {
-            error_setg(errp, "cssid %hhx not valid for %s devices",
-                       bus_id.cssid,
-                       (is_virtual ? "virtual" : "non-virtual"));
-            return NULL;
-        }
-    }
-    if (bus_id.valid) {
         if (squash_mcss) {
             bus_id.cssid = channel_subsys.default_cssid;
         } else if (!channel_subsys.css[bus_id.cssid]) {
@@ -2396,19 +2386,8 @@ SubchDev *css_create_sch(CssDevId bus_id, bool 
is_virtual, bool squash_mcss,
                                            bus_id.devid, &schid, errp)) {
             return NULL;
-    } else if (squash_mcss || is_virtual) {
-        bus_id.cssid = channel_subsys.default_cssid;
-        if (!css_find_free_subch_and_devno(bus_id.cssid, &bus_id.ssid,
-                                           &bus_id.devid, &schid, errp)) {
-            return NULL;
-        }
     } else {
-        for (bus_id.cssid = 0; bus_id.cssid < MAX_CSSID; ++bus_id.cssid) {
-            if (bus_id.cssid == VIRTUAL_CSSID) {
-                continue;
-            }
+        for (bus_id.cssid = channel_subsys.default_cssid;;) {
             if (!channel_subsys.css[bus_id.cssid]) {
                 css_create_css_image(bus_id.cssid, false);
@@ -2418,7 +2397,8 @@ SubchDev *css_create_sch(CssDevId bus_id, bool 
is_virtual, bool squash_mcss,
                                                 NULL)) {
-            if (bus_id.cssid == MAX_CSSID) {
+            bus_id.cssid = (bus_id.cssid + 1) % MAX_CSSID;
+            if (bus_id.cssid == channel_subsys.default_cssid) {
                 error_setg(errp, "Virtual channel subsystem is full!");
                 return NULL;
diff --git a/hw/s390x/s390-ccw.c b/hw/s390x/s390-ccw.c
index 0ef232ec27..4a9d4d2534 100644
--- a/hw/s390x/s390-ccw.c
+++ b/hw/s390x/s390-ccw.c
@@ -77,7 +77,7 @@ static void s390_ccw_realize(S390CCWDevice *cdev, char 
*sysfsdev, Error **errp)
         goto out_err_propagate;
-    sch = css_create_sch(ccw_dev->devno, false, cbus->squash_mcss, &err);
+    sch = css_create_sch(ccw_dev->devno, cbus->squash_mcss, &err);
     if (!sch) {
         goto out_mdevid_free;
diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 6a57f94197..b558b2adad 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -556,6 +556,21 @@ static inline void machine_set_squash_mcss(Object *obj, 
bool value,
     ms->s390_squash_mcss = value;
+static bool prop_get_true(Object *obj, Error **errp)
+    return true;
+ * This is a fake setter so the device shows up in the output of
+ * --machine s390-ccw-virtio,help.
+ */
+static inline void prop_set_bool_fail(Object *obj, bool value,
+                                           Error **errp)
+    error_setg(errp, "Tried to set non-settable property!");
 static inline void s390_machine_initfn(Object *obj)
     object_property_add_bool(obj, "aes-key-wrap",
@@ -587,6 +602,12 @@ static inline void s390_machine_initfn(Object *obj)
             "enable/disable squashing subchannels into the default css",
     object_property_set_bool(obj, false, "s390-squash-mcss", NULL);
+    object_property_add_bool(obj, "cssid-unrestricted",
+                                   prop_get_true, prop_set_bool_fail, NULL);
+    object_property_set_description(obj, "cssid-unrestricted",
+            "can use any cssid with any css device (the restriction,"
+            " 0xfe for virtual and 0x00-0xfd for non-virtual, is no more)",
+            NULL);
 static const TypeInfo ccw_machine_info = {
diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index 184515ce94..3dd902a664 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -701,7 +701,7 @@ static void virtio_ccw_device_realize(VirtioCcwDevice *dev, 
Error **errp)
     SubchDev *sch;
     Error *err = NULL;
-    sch = css_create_sch(ccw_dev->devno, true, cbus->squash_mcss, errp);
+    sch = css_create_sch(ccw_dev->devno, cbus->squash_mcss, errp);
     if (!sch) {
diff --git a/include/hw/s390x/css.h b/include/hw/s390x/css.h
index ab6ebe66b5..53c270a216 100644
--- a/include/hw/s390x/css.h
+++ b/include/hw/s390x/css.h
@@ -272,12 +272,9 @@ extern const PropertyInfo css_devid_ro_propinfo;
  * default css image for it.
  * If @p bus_id is valid, and @p squash_mcss is false, verify that it is
  * not already in use, and find a free devno for it.
- * If @p bus_id is not valid, and if either @p squash_mcss or @p is_virtual
- * is true, find a free subchannel id and device number across all
- * subchannel sets from the default css image.
- * If @p bus_id is not valid, and if both @p squash_mcss and @p is_virtual
- * are false, find a non-full css image and find a free subchannel id and
- * device number across all subchannel sets from it.
+ * If @p bus_id is not valid find a free subchannel id and device number
+ * across all subchannel sets and all css images starting from the default
+ * css image.
  * If either of the former actions succeed, allocate a subchannel structure,
  * initialise it with the bus id, subchannel id and device number, register
@@ -286,8 +283,7 @@ extern const PropertyInfo css_devid_ro_propinfo;
  * The caller becomes owner of the returned subchannel structure and
  * is responsible for unregistering and freeing it.
-SubchDev *css_create_sch(CssDevId bus_id, bool is_virtual, bool squash_mcss,
-                         Error **errp);
+SubchDev *css_create_sch(CssDevId bus_id, bool squash_mcss, Error **errp);
 /** Turn on css migration */
 void css_register_vmstate(void);
diff --git a/util/qemu-config.c b/util/qemu-config.c
index 99b0e46fa3..acfc452fc2 100644
--- a/util/qemu-config.c
+++ b/util/qemu-config.c
@@ -233,6 +233,12 @@ static QemuOptsList machine_opts = {
             .help = "Up to 8 chars in set of [A-Za-z0-9. ](lower case chars"
                     " converted to upper case) to pass to machine"
                     " loader, boot manager, and guest kernel",
+        },{
+            /* TODO: Consider device property. This is way too ugly. */
+            .name = "cssid-unrestricted",
+            .type = QEMU_OPT_BOOL,
+            .help = "can use any cssid with any css device (the restriction,"
+            " 0xfe for virtual and 0x00-0xfd for non-virtual, is no more)",
         { /* End of list */ }

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