[RFC PATCH v12 0/4] Report vfio-ap configuration changes

[RFC PATCH v12 0/4] Report vfio-ap configuration changes

[Rorie Reyes]

Changelog:
v12: 
- adding locks to 'ap_chsc_sei_nt0_have_event' and 'ap_chsc_sei_nt0_get_event'

v11:
- reverted return type to int for 'ap_chsc_sei_nt0_get_event'
   - files reflected are 'ap.c', 'ap-bridge'h, and 'ap-stub.c'
- using defined variables to represent return 0 and 1 to reflect logical sense
- update documentation for 'ap_chsc_sei_nt0_get_event' to reflect return types

v10:
- Added reviewed by for patch 4 by Tony and Cedric
   - review needed for patch 2 and 3
- changed 'cfg_chg_events' to a static variable
- locked 'cfg_chg_events' using QemuMutex
- removed ';' at the end of the definition for NT0
- returning bools for 'ap_chsc_sei_nt0_get_event' and
'ap_chsc_sei_nt0_have_event'
- updated the header file that contains 'ap_chsc_sei_nt0_get_event' and
'ap_chsc_sei_nt0_have_event' to a bool function
- added documentation explaining the returning bool functions
- whitespace clean up

v9:
- added SPDX licensing to newly created file 'hw/s390x/ap-stub.c'

v8:
- fixed windows cross-compile build
- moved /hw/vfio/ap-stub.c to /hw/s390x/ap-stub.c
- updated the use of stub file to MAINTAINERS to reflect new location
- removed if_false for 'CONFIG_VFIO_AP' statement from /hw/vfio/meson.build
- added if_false for 'CONFIG_VFIO_AP' to use ap-stub.c in /hw/s390x/meson.build
- all those changes still address '--without-default-devices' issue from v5

v7:
- Dropped initial commit for linux-header file vfio.h since I created two new commits
to address the changes made in v6
- Moved patches 6 and 7 to the beginning of the series after dropping the first patch
   - Because I dropped the initial commit for linux-header file vfio.h, I had to add
VFIO_AP_CFG_CHG_IRQ_INDEX
- Resyncing latest to v6.15-rc3
- Still need Thomas Huth's review of v5 changes for patch 6/6

v6:
- Updating the update-linux-headers script to address kernel commit change 8a14
- Update headers to retrieve uapi information for vfio-ap for update to Linux v6.15-rc1
- Still need Thomas Huth's review of v5 changes for patch 7/7 (see below)

v5:
- configuring using the '--without-default-devices' fails when building the code
- created a stub file for functions ap_chsc_sei_nt0_get_event and ap_chsc_sei_nt0_have_event
- add if_false for 'CONFIG_VFIO_AP' use ap-stub.c in meson.build
- add the use of the stub file to MAINTAINERS since it's a new file

v4:
- allocating cfg_chg_event before inserting into the queue
- calling nt0_have_event in if loop to check if there are any
elemenets in the queue, then calling QTAILQ_FIRST when the check
passes
- moving memset() after the check

v3:
- changes that were made to patch 3/5 should have been made in
patch 2/5

v2:
- removed warnings that weren't needed
- added unregister function
- removed whitelines
- changed variable names for consistency
- removed rc variable and returning 1 or 0 outright
- reversed logics for if statements
- using g_free() instead of free()
- replaced hardcoded numeric values by defining them with #define
in the header

--------------------------------------------------------------------------

Rorie Reyes (4):
  hw/vfio/ap: notification handler for AP config changed event
  hw/vfio/ap: store object indicating AP config changed in a queue
  hw/vfio/ap: Storing event information for an AP configuration change
    event
  s390: implementing CHSC SEI for AP config change

 MAINTAINERS                  |  1 +
 hw/s390x/ap-stub.c           | 21 +++++++++
 hw/s390x/meson.build         |  1 +
 hw/vfio/ap.c                 | 86 ++++++++++++++++++++++++++++++++++++
 include/hw/s390x/ap-bridge.h | 39 ++++++++++++++++
 target/s390x/ioinst.c        | 11 ++++-
 6 files changed, 157 insertions(+), 2 deletions(-)
 create mode 100644 hw/s390x/ap-stub.c

-- 
2.48.1

[RFC PATCH v12 1/4] hw/vfio/ap: notification handler for AP config changed event

[Rorie Reyes]

Register an event notifier handler to process AP configuration
change events by queuing the event and generating a CRW to let
the guest know its AP configuration has changed

Signed-off-by: Rorie Reyes <rreyes@linux.ibm.com>
Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>
---
 hw/vfio/ap.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/hw/vfio/ap.c b/hw/vfio/ap.c
index 785c0a0197..93c74ebedb 100644
--- a/hw/vfio/ap.c
+++ b/hw/vfio/ap.c
@@ -18,6 +18,7 @@
 #include "hw/vfio/vfio-device.h"
 #include "system/iommufd.h"
 #include "hw/s390x/ap-device.h"
+#include "hw/s390x/css.h"
 #include "qemu/error-report.h"
 #include "qemu/event_notifier.h"
 #include "qemu/main-loop.h"
@@ -37,6 +38,7 @@ struct VFIOAPDevice {
     APDevice apdev;
     VFIODevice vdev;
     EventNotifier req_notifier;
+    EventNotifier cfg_notifier;
 };
 
 OBJECT_DECLARE_SIMPLE_TYPE(VFIOAPDevice, VFIO_AP_DEVICE)
@@ -70,6 +72,18 @@ static void vfio_ap_req_notifier_handler(void *opaque)
     }
 }
 
+static void vfio_ap_cfg_chg_notifier_handler(void *opaque)
+{
+    VFIOAPDevice *vapdev = opaque;
+
+    if (!event_notifier_test_and_clear(&vapdev->cfg_notifier)) {
+        return;
+    }
+
+    css_generate_css_crws(0);
+
+}
+
 static bool vfio_ap_register_irq_notifier(VFIOAPDevice *vapdev,
                                           unsigned int irq, Error **errp)
 {
@@ -85,6 +99,10 @@ static bool vfio_ap_register_irq_notifier(VFIOAPDevice *vapdev,
         notifier = &vapdev->req_notifier;
         fd_read = vfio_ap_req_notifier_handler;
         break;
+    case VFIO_AP_CFG_CHG_IRQ_INDEX:
+        notifier = &vapdev->cfg_notifier;
+        fd_read = vfio_ap_cfg_chg_notifier_handler;
+        break;
     default:
         error_setg(errp, "vfio: Unsupported device irq(%d)", irq);
         return false;
@@ -137,6 +155,9 @@ static void vfio_ap_unregister_irq_notifier(VFIOAPDevice *vapdev,
     case VFIO_AP_REQ_IRQ_INDEX:
         notifier = &vapdev->req_notifier;
         break;
+    case VFIO_AP_CFG_CHG_IRQ_INDEX:
+        notifier = &vapdev->cfg_notifier;
+        break;
     default:
         error_report("vfio: Unsupported device irq(%d)", irq);
         return;
@@ -176,6 +197,15 @@ static void vfio_ap_realize(DeviceState *dev, Error **errp)
         warn_report_err(err);
     }
 
+    if (!vfio_ap_register_irq_notifier(vapdev, VFIO_AP_CFG_CHG_IRQ_INDEX, &err))
+    {
+        /*
+         * Report this error, but do not make it a failing condition.
+         * Lack of this IRQ in the host does not prevent normal operation.
+         */
+        warn_report_err(err);
+    }
+
     return;
 
 error:
@@ -188,6 +218,7 @@ static void vfio_ap_unrealize(DeviceState *dev)
     VFIOAPDevice *vapdev = VFIO_AP_DEVICE(dev);
 
     vfio_ap_unregister_irq_notifier(vapdev, VFIO_AP_REQ_IRQ_INDEX);
+    vfio_ap_unregister_irq_notifier(vapdev, VFIO_AP_CFG_CHG_IRQ_INDEX);
     vfio_device_detach(&vapdev->vdev);
     g_free(vapdev->vdev.name);
 }
-- 
2.48.1

[RFC PATCH v12 2/4] hw/vfio/ap: store object indicating AP config changed in a queue

[Rorie Reyes]

Creates an object indicating that an AP configuration change event
has been received and stores it in a queue. These objects will later
be used to store event information for an AP configuration change
when the CHSC instruction is intercepted.

Signed-off-by: Rorie Reyes <rreyes@linux.ibm.com>
---
 hw/vfio/ap.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/hw/vfio/ap.c b/hw/vfio/ap.c
index 93c74ebedb..fc435f5c5b 100644
--- a/hw/vfio/ap.c
+++ b/hw/vfio/ap.c
@@ -41,6 +41,13 @@ struct VFIOAPDevice {
     EventNotifier cfg_notifier;
 };
 
+typedef struct APConfigChgEvent {
+    QTAILQ_ENTRY(APConfigChgEvent) next;
+} APConfigChgEvent;
+
+static QTAILQ_HEAD(, APConfigChgEvent) cfg_chg_events =
+    QTAILQ_HEAD_INITIALIZER(cfg_chg_events);
+
 OBJECT_DECLARE_SIMPLE_TYPE(VFIOAPDevice, VFIO_AP_DEVICE)
 
 static void vfio_ap_compute_needs_reset(VFIODevice *vdev)
@@ -74,12 +81,17 @@ static void vfio_ap_req_notifier_handler(void *opaque)
 
 static void vfio_ap_cfg_chg_notifier_handler(void *opaque)
 {
+    APConfigChgEvent *cfg_chg_event;
     VFIOAPDevice *vapdev = opaque;
 
     if (!event_notifier_test_and_clear(&vapdev->cfg_notifier)) {
         return;
     }
 
+    cfg_chg_event = g_new0(APConfigChgEvent, 1);
+
+    QTAILQ_INSERT_TAIL(&cfg_chg_events, cfg_chg_event, next);
+
     css_generate_css_crws(0);
 
 }
-- 
2.48.1

Re: [RFC PATCH v12 2/4] hw/vfio/ap: store object indicating AP config changed in a queue

[Anthony Krowiak]

On 6/6/25 2:37 PM, Rorie Reyes wrote:
> Creates an object indicating that an AP configuration change event
> has been received and stores it in a queue. These objects will later
> be used to store event information for an AP configuration change
> when the CHSC instruction is intercepted.
>
> Signed-off-by: Rorie Reyes <rreyes@linux.ibm.com>

See comment below. With that change made:

Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>

> ---
>   hw/vfio/ap.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
>
> diff --git a/hw/vfio/ap.c b/hw/vfio/ap.c
> index 93c74ebedb..fc435f5c5b 100644
> --- a/hw/vfio/ap.c
> +++ b/hw/vfio/ap.c
> @@ -41,6 +41,13 @@ struct VFIOAPDevice {
>       EventNotifier cfg_notifier;
>   };
>   
> +typedef struct APConfigChgEvent {
> +    QTAILQ_ENTRY(APConfigChgEvent) next;
> +} APConfigChgEvent;
> +
> +static QTAILQ_HEAD(, APConfigChgEvent) cfg_chg_events =
> +    QTAILQ_HEAD_INITIALIZER(cfg_chg_events);
> +
>   OBJECT_DECLARE_SIMPLE_TYPE(VFIOAPDevice, VFIO_AP_DEVICE)
>   
>   static void vfio_ap_compute_needs_reset(VFIODevice *vdev)
> @@ -74,12 +81,17 @@ static void vfio_ap_req_notifier_handler(void *opaque)
>   
>   static void vfio_ap_cfg_chg_notifier_handler(void *opaque)
>   {
> +    APConfigChgEvent *cfg_chg_event;
>       VFIOAPDevice *vapdev = opaque;
>   
>       if (!event_notifier_test_and_clear(&vapdev->cfg_notifier)) {
>           return;
>       }
>   
> +    cfg_chg_event = g_new0(APConfigChgEvent, 1);
> +
> +    QTAILQ_INSERT_TAIL(&cfg_chg_events, cfg_chg_event, next);

Need locking here:

WITH_QEMU_LOCK_GUARD(&cfg_chg_events_lock) {

QTAILQ_INSERT_TAIL(&cfg_chg_events, cfg_chg_event, next);

}



> +
>       css_generate_css_crws(0);
>   
>   }

Re: [RFC PATCH v12 2/4] hw/vfio/ap: store object indicating AP config changed in a queue

[Rorie Reyes]

[-- Attachment #1: Type: text/plain, Size: 384 bytes --]


On 6/9/25 6:19 AM, Anthony Krowiak wrote:
>> + QTAILQ_INSERT_TAIL(&cfg_chg_events, cfg_chg_event, next);
>
> Need locking here:
>
> WITH_QEMU_LOCK_GUARD(&cfg_chg_events_lock) {
>
> QTAILQ_INSERT_TAIL(&cfg_chg_events, cfg_chg_event, next);
>
> }
Wouldn't QEMU_LOCK_GUARD(&cfg_chg_events_lock) be better since it's a 
one line? Or the fact there's no return here makes it insufficient?

[-- Attachment #2: Type: text/html, Size: 924 bytes --]

[RFC PATCH v12 3/4] hw/vfio/ap: Storing event information for an AP configuration change event

[Rorie Reyes]

These functions can be invoked by the function that handles interception
of the CHSC SEI instruction for requests indicating the accessibility of
one or more adjunct processors has changed.

Signed-off-by: Rorie Reyes <rreyes@linux.ibm.com>
---
 hw/vfio/ap.c                 | 43 ++++++++++++++++++++++++++++++++++++
 include/hw/s390x/ap-bridge.h | 39 ++++++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+)

diff --git a/hw/vfio/ap.c b/hw/vfio/ap.c
index fc435f5c5b..7e3c6278b3 100644
--- a/hw/vfio/ap.c
+++ b/hw/vfio/ap.c
@@ -10,6 +10,7 @@
  * directory.
  */
 
+#include <stdbool.h>
 #include "qemu/osdep.h"
 #include CONFIG_DEVICES /* CONFIG_IOMMUFD */
 #include <linux/vfio.h>
@@ -21,6 +22,7 @@
 #include "hw/s390x/css.h"
 #include "qemu/error-report.h"
 #include "qemu/event_notifier.h"
+#include "qemu/lockable.h"
 #include "qemu/main-loop.h"
 #include "qemu/module.h"
 #include "qemu/option.h"
@@ -48,6 +50,8 @@ typedef struct APConfigChgEvent {
 static QTAILQ_HEAD(, APConfigChgEvent) cfg_chg_events =
     QTAILQ_HEAD_INITIALIZER(cfg_chg_events);
 
+static QemuMutex cfg_chg_events_lock;
+
 OBJECT_DECLARE_SIMPLE_TYPE(VFIOAPDevice, VFIO_AP_DEVICE)
 
 static void vfio_ap_compute_needs_reset(VFIODevice *vdev)
@@ -96,6 +100,38 @@ static void vfio_ap_cfg_chg_notifier_handler(void *opaque)
 
 }
 
+int ap_chsc_sei_nt0_get_event(void *res)
+{
+    ChscSeiNt0Res *nt0_res  = (ChscSeiNt0Res *)res;
+    APConfigChgEvent *cfg_chg_event;
+
+    WITH_QEMU_LOCK_GUARD(&cfg_chg_events_lock) {
+        if (QTAILQ_EMPTY(&cfg_chg_events)) {
+            return EVENT_INFORMATION_NOT_STORED;
+        }
+
+        cfg_chg_event = QTAILQ_FIRST(&cfg_chg_events);
+        QTAILQ_REMOVE(&cfg_chg_events, cfg_chg_event, next);
+    }
+
+    memset(nt0_res, 0, sizeof(*nt0_res));
+    g_free(cfg_chg_event);
+    nt0_res->flags |= PENDING_EVENT_INFO_BITMASK;
+    nt0_res->length = sizeof(ChscSeiNt0Res);
+    nt0_res->code = NT0_RES_RESPONSE_CODE;
+    nt0_res->nt = NT0_RES_NT_DEFAULT;
+    nt0_res->rs = NT0_RES_RS_AP_CHANGE;
+    nt0_res->cc = NT0_RES_CC_AP_CHANGE;
+
+    return EVENT_INFORMATION_STORED;
+}
+
+bool ap_chsc_sei_nt0_have_event(void)
+{
+    QEMU_LOCK_GUARD(&cfg_chg_events_lock);
+    return !QTAILQ_EMPTY(&cfg_chg_events);
+}
+
 static bool vfio_ap_register_irq_notifier(VFIOAPDevice *vapdev,
                                           unsigned int irq, Error **errp)
 {
@@ -192,6 +228,13 @@ static void vfio_ap_realize(DeviceState *dev, Error **errp)
     VFIOAPDevice *vapdev = VFIO_AP_DEVICE(dev);
     VFIODevice *vbasedev = &vapdev->vdev;
 
+    static bool lock_initialized;
+
+    if (!lock_initialized) {
+        qemu_mutex_init(&cfg_chg_events_lock);
+        lock_initialized = true;
+    }
+
     if (!vfio_device_get_name(vbasedev, errp)) {
         return;
     }
diff --git a/include/hw/s390x/ap-bridge.h b/include/hw/s390x/ap-bridge.h
index 470e439a98..7efc52928d 100644
--- a/include/hw/s390x/ap-bridge.h
+++ b/include/hw/s390x/ap-bridge.h
@@ -16,4 +16,43 @@
 
 void s390_init_ap(void);
 
+typedef struct ChscSeiNt0Res {
+    uint16_t length;
+    uint16_t code;
+    uint8_t reserved1;
+    uint16_t reserved2;
+    uint8_t nt;
+#define PENDING_EVENT_INFO_BITMASK 0x80;
+    uint8_t flags;
+    uint8_t reserved3;
+    uint8_t rs;
+    uint8_t cc;
+} QEMU_PACKED ChscSeiNt0Res;
+
+#define NT0_RES_RESPONSE_CODE 1
+#define NT0_RES_NT_DEFAULT    0
+#define NT0_RES_RS_AP_CHANGE  5
+#define NT0_RES_CC_AP_CHANGE  3
+
+#define EVENT_INFORMATION_NOT_STORED 1
+#define EVENT_INFORMATION_STORED     0
+
+/**
+ * ap_chsc_sei_nt0_get_event - Retrieve the next pending AP config
+ * change event
+ * @res: Pointer to a ChscSeiNt0Res struct to be filled with event
+ * data
+ *
+ * This function checks for any pending AP config change events and,
+ * if present, populates the provided response structure with the
+ * appropriate SEI NT0 fields.
+ *
+ * Return:
+ *   EVENT_INFORMATION_STORED - An event was available and written to @res
+ *   EVENT_INFORMATION_NOT_STORED - No event was available
+ */
+int ap_chsc_sei_nt0_get_event(void *res);
+
+bool ap_chsc_sei_nt0_have_event(void);
+
 #endif
-- 
2.48.1

Re: [RFC PATCH v12 3/4] hw/vfio/ap: Storing event information for an AP configuration change event

[Anthony Krowiak]

On 6/6/25 2:37 PM, Rorie Reyes wrote:
> These functions can be invoked by the function that handles interception
> of the CHSC SEI instruction for requests indicating the accessibility of
> one or more adjunct processors has changed.
>
> Signed-off-by: Rorie Reyes <rreyes@linux.ibm.com>

Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>

> ---
>   hw/vfio/ap.c                 | 43 ++++++++++++++++++++++++++++++++++++
>   include/hw/s390x/ap-bridge.h | 39 ++++++++++++++++++++++++++++++++
>   2 files changed, 82 insertions(+)
>
> diff --git a/hw/vfio/ap.c b/hw/vfio/ap.c
> index fc435f5c5b..7e3c6278b3 100644
> --- a/hw/vfio/ap.c
> +++ b/hw/vfio/ap.c
> @@ -10,6 +10,7 @@
>    * directory.
>    */
>   
> +#include <stdbool.h>
>   #include "qemu/osdep.h"
>   #include CONFIG_DEVICES /* CONFIG_IOMMUFD */
>   #include <linux/vfio.h>
> @@ -21,6 +22,7 @@
>   #include "hw/s390x/css.h"
>   #include "qemu/error-report.h"
>   #include "qemu/event_notifier.h"
> +#include "qemu/lockable.h"
>   #include "qemu/main-loop.h"
>   #include "qemu/module.h"
>   #include "qemu/option.h"
> @@ -48,6 +50,8 @@ typedef struct APConfigChgEvent {
>   static QTAILQ_HEAD(, APConfigChgEvent) cfg_chg_events =
>       QTAILQ_HEAD_INITIALIZER(cfg_chg_events);
>   
> +static QemuMutex cfg_chg_events_lock;
> +
>   OBJECT_DECLARE_SIMPLE_TYPE(VFIOAPDevice, VFIO_AP_DEVICE)
>   
>   static void vfio_ap_compute_needs_reset(VFIODevice *vdev)
> @@ -96,6 +100,38 @@ static void vfio_ap_cfg_chg_notifier_handler(void *opaque)
>   
>   }
>   
> +int ap_chsc_sei_nt0_get_event(void *res)
> +{
> +    ChscSeiNt0Res *nt0_res  = (ChscSeiNt0Res *)res;
> +    APConfigChgEvent *cfg_chg_event;
> +
> +    WITH_QEMU_LOCK_GUARD(&cfg_chg_events_lock) {
> +        if (QTAILQ_EMPTY(&cfg_chg_events)) {
> +            return EVENT_INFORMATION_NOT_STORED;
> +        }
> +
> +        cfg_chg_event = QTAILQ_FIRST(&cfg_chg_events);
> +        QTAILQ_REMOVE(&cfg_chg_events, cfg_chg_event, next);
> +    }
> +
> +    memset(nt0_res, 0, sizeof(*nt0_res));
> +    g_free(cfg_chg_event);
> +    nt0_res->flags |= PENDING_EVENT_INFO_BITMASK;
> +    nt0_res->length = sizeof(ChscSeiNt0Res);
> +    nt0_res->code = NT0_RES_RESPONSE_CODE;
> +    nt0_res->nt = NT0_RES_NT_DEFAULT;
> +    nt0_res->rs = NT0_RES_RS_AP_CHANGE;
> +    nt0_res->cc = NT0_RES_CC_AP_CHANGE;
> +
> +    return EVENT_INFORMATION_STORED;
> +}
> +
> +bool ap_chsc_sei_nt0_have_event(void)
> +{
> +    QEMU_LOCK_GUARD(&cfg_chg_events_lock);
> +    return !QTAILQ_EMPTY(&cfg_chg_events);
> +}
> +
>   static bool vfio_ap_register_irq_notifier(VFIOAPDevice *vapdev,
>                                             unsigned int irq, Error **errp)
>   {
> @@ -192,6 +228,13 @@ static void vfio_ap_realize(DeviceState *dev, Error **errp)
>       VFIOAPDevice *vapdev = VFIO_AP_DEVICE(dev);
>       VFIODevice *vbasedev = &vapdev->vdev;
>   
> +    static bool lock_initialized;
> +
> +    if (!lock_initialized) {
> +        qemu_mutex_init(&cfg_chg_events_lock);
> +        lock_initialized = true;
> +    }
> +
>       if (!vfio_device_get_name(vbasedev, errp)) {
>           return;
>       }
> diff --git a/include/hw/s390x/ap-bridge.h b/include/hw/s390x/ap-bridge.h
> index 470e439a98..7efc52928d 100644
> --- a/include/hw/s390x/ap-bridge.h
> +++ b/include/hw/s390x/ap-bridge.h
> @@ -16,4 +16,43 @@
>   
>   void s390_init_ap(void);
>   
> +typedef struct ChscSeiNt0Res {
> +    uint16_t length;
> +    uint16_t code;
> +    uint8_t reserved1;
> +    uint16_t reserved2;
> +    uint8_t nt;
> +#define PENDING_EVENT_INFO_BITMASK 0x80;
> +    uint8_t flags;
> +    uint8_t reserved3;
> +    uint8_t rs;
> +    uint8_t cc;
> +} QEMU_PACKED ChscSeiNt0Res;
> +
> +#define NT0_RES_RESPONSE_CODE 1
> +#define NT0_RES_NT_DEFAULT    0
> +#define NT0_RES_RS_AP_CHANGE  5
> +#define NT0_RES_CC_AP_CHANGE  3
> +
> +#define EVENT_INFORMATION_NOT_STORED 1
> +#define EVENT_INFORMATION_STORED     0
> +
> +/**
> + * ap_chsc_sei_nt0_get_event - Retrieve the next pending AP config
> + * change event
> + * @res: Pointer to a ChscSeiNt0Res struct to be filled with event
> + * data
> + *
> + * This function checks for any pending AP config change events and,
> + * if present, populates the provided response structure with the
> + * appropriate SEI NT0 fields.
> + *
> + * Return:
> + *   EVENT_INFORMATION_STORED - An event was available and written to @res
> + *   EVENT_INFORMATION_NOT_STORED - No event was available
> + */
> +int ap_chsc_sei_nt0_get_event(void *res);
> +
> +bool ap_chsc_sei_nt0_have_event(void);
> +
>   #endif

[RFC PATCH v12 4/4] s390: implementing CHSC SEI for AP config change

[Rorie Reyes]

Handle interception of the CHSC SEI instruction for requests
indicating the guest's AP configuration has changed.

If configuring --without-default-devices, hw/s390x/ap-stub.c
was created to handle such circumstance. Also added the
following to hw/s390x/meson.build if CONFIG_VFIO_AP is
false, it will use the stub file.

Signed-off-by: Rorie Reyes <rreyes@linux.ibm.com>
Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
---
 MAINTAINERS           |  1 +
 hw/s390x/ap-stub.c    | 21 +++++++++++++++++++++
 hw/s390x/meson.build  |  1 +
 target/s390x/ioinst.c | 11 +++++++++--
 4 files changed, 32 insertions(+), 2 deletions(-)
 create mode 100644 hw/s390x/ap-stub.c

diff --git a/MAINTAINERS b/MAINTAINERS
index aa6763077e..1e84bfeaee 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -112,6 +112,7 @@ F: hw/intc/s390_flic.c
 F: hw/intc/s390_flic_kvm.c
 F: hw/s390x/
 F: hw/vfio/ap.c
+F: hw/s390x/ap-stub.c
 F: hw/vfio/ccw.c
 F: hw/watchdog/wdt_diag288.c
 F: include/hw/s390x/
diff --git a/hw/s390x/ap-stub.c b/hw/s390x/ap-stub.c
new file mode 100644
index 0000000000..001fe5f8b0
--- /dev/null
+++ b/hw/s390x/ap-stub.c
@@ -0,0 +1,21 @@
+/*
+ * VFIO based AP matrix device assignment
+ *
+ * Copyright 2025 IBM Corp.
+ * Author(s): Rorie Reyes <rreyes@linux.ibm.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "hw/s390x/ap-bridge.h"
+
+int ap_chsc_sei_nt0_get_event(void *res)
+{
+    return EVENT_INFORMATION_NOT_STORED;
+}
+
+bool ap_chsc_sei_nt0_have_event(void)
+{
+    return false;
+}
diff --git a/hw/s390x/meson.build b/hw/s390x/meson.build
index 3bbebfd817..99cbcbd7d6 100644
--- a/hw/s390x/meson.build
+++ b/hw/s390x/meson.build
@@ -33,6 +33,7 @@ s390x_ss.add(when: 'CONFIG_S390_CCW_VIRTIO', if_true: files(
 ))
 s390x_ss.add(when: 'CONFIG_TERMINAL3270', if_true: files('3270-ccw.c'))
 s390x_ss.add(when: 'CONFIG_VFIO', if_true: files('s390-pci-vfio.c'))
+s390x_ss.add(when: 'CONFIG_VFIO_AP', if_false: files('ap-stub.c'))
 
 virtio_ss = ss.source_set()
 virtio_ss.add(files('virtio-ccw.c'))
diff --git a/target/s390x/ioinst.c b/target/s390x/ioinst.c
index fe62ba5b06..2320dd4c12 100644
--- a/target/s390x/ioinst.c
+++ b/target/s390x/ioinst.c
@@ -18,6 +18,7 @@
 #include "trace.h"
 #include "hw/s390x/s390-pci-bus.h"
 #include "target/s390x/kvm/pv.h"
+#include "hw/s390x/ap-bridge.h"
 
 /* All I/O instructions but chsc use the s format */
 static uint64_t get_address_from_regs(CPUS390XState *env, uint32_t ipb,
@@ -574,13 +575,19 @@ out:
 
 static int chsc_sei_nt0_get_event(void *res)
 {
-    /* no events yet */
+    if (s390_has_feat(S390_FEAT_AP)) {
+        return ap_chsc_sei_nt0_get_event(res);
+    }
+
     return 1;
 }
 
 static int chsc_sei_nt0_have_event(void)
 {
-    /* no events yet */
+    if (s390_has_feat(S390_FEAT_AP)) {
+        return ap_chsc_sei_nt0_have_event();
+    }
+
     return 0;
 }
 
-- 
2.48.1