Re: [RFC PATCH v11 3/4] hw/vfio/ap: Storing event information for an AP configuration change event

[Rorie Reyes <rreyes@linux.ibm.com>]

On 6/4/25 9:47 AM, Anthony Krowiak wrote:
>
>
>
> On 6/3/25 4:30 PM, Cédric Le Goater wrote:
>> On 6/3/25 20:01, Rorie Reyes wrote:
>>>
>>> On 6/3/25 10:21 AM, Cédric Le Goater wrote:
>>>> On 6/3/25 14:58, Rorie Reyes wrote:
>>>>> Hey Cedric,
>>>>>
>>>>> You mentioned the following in my v9 patches
>>>>>
>>>>> "In that case, let's keep it simple (no mutex) and add a 
>>>>> assert(bql_locked())
>>>>> statement where we think the bql should be protecting access to 
>>>>> shared
>>>>> resources. "
>>>>>
>>>>> Does this still apply down bellow?
>>>>
>>>> Anthony replied :
>>>>
>>>> https://lore.kernel.org/qemu-devel/ed2a2aa3-68a7-480c-a6a4-a8219af12d7b@linux.ibm.com/ 
>>>>
>>>>
>>>> Thanks,
>>>>
>>>> C.
>>>>
>>> So we'll still use WITH_QEMU_LOCK_GUARD?
>>
>> If a lock is needed to protect the list, then 
>> ap_chsc_sei_nt0_have_event()
>> should lock/unlock too. WITH_QEMU_LOCK_GUARD() is just a pratical way to
>> do so.
>
> Since ap_chsc_sei_nt0_have_event() is a single line that returns
> !QTAILQ_EMPTY(&cfg_chg_events), wouldn't it be better to just
> use the QEMU_LOCK_GUARD macro which, if I'm not mistaken,
> will unlock on the return statement?
>>
>>
>> Thanks,
>>
>> C.
>>
>>
>>
>>>>>
>>>>> On 5/26/25 4:40 AM, Cédric Le Goater wrote:
>>>>>> On 5/23/25 18:03, 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>
>>>>>>> ---
>>>>>>>   hw/vfio/ap.c                 | 53 
>>>>>>> ++++++++++++++++++++++++++++++++++++
>>>>>>>   include/hw/s390x/ap-bridge.h | 39 ++++++++++++++++++++++++++
>>>>>>>   2 files changed, 92 insertions(+)
>>>>>>>
>>>>>>> diff --git a/hw/vfio/ap.c b/hw/vfio/ap.c
>>>>>>> index fc435f5c5b..97a42a575a 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>
>>>>>>> @@ -48,6 +49,8 @@ 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 +99,49 @@ 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;
>>>>>>> +
>>>>>>> +    qemu_mutex_lock(&cfg_chg_events_lock);
>>>>>>
>>>>>> please consider using WITH_QEMU_LOCK_GUARD()
>>>>>>
>>>>> See note above about bql_locked
>>>>>>> +    if (!ap_chsc_sei_nt0_have_event()) {
>>>>>>> +        qemu_mutex_unlock(&cfg_chg_events_lock);
>>>>>>> +        return EVENT_INFORMATION_NOT_STORED;
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    cfg_chg_event = QTAILQ_FIRST(&cfg_chg_events);
>>>>>>> +    QTAILQ_REMOVE(&cfg_chg_events, cfg_chg_event, next);
>>>>>>> +
>>>>>>> +    qemu_mutex_unlock(&cfg_chg_events_lock);
>>>>>>> +
>>>>>>> +    memset(nt0_res, 0, sizeof(*nt0_res));
>>>>>>> +    g_free(cfg_chg_event);
>>>>>>> +
>>>>>>> +    /*
>>>>>>> +     * If there are any AP configuration change events in the 
>>>>>>> queue,
>>>>>>> +     * indicate to the caller that there is pending event info in
>>>>>>> +     * the response block
>>>>>>> +     */
>>>>>>> +    if (ap_chsc_sei_nt0_have_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)
>>>>>>
>>>>>> hmm, no locking ?
>>>>>>
How important do we need to lock this? When I lock this method my guest 
freezes every time. But when I only lock the get event, my code 
continues to work as designed
>>>>> See not above for bql_locked
>>>>>>> +{
>>>>>>> +    return !QTAILQ_EMPTY(&cfg_chg_events);
>>>>>>> +}
>>>>>>> +
>>>>>>>   static bool vfio_ap_register_irq_notifier(VFIOAPDevice *vapdev,
>>>>>>>                                             unsigned int irq, 
>>>>>>> Error **errp)