On 6/9/25 12:44 PM, Rorie Reyes wrote:
> 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 <[email protected]>
> Reviewed-by: Anthony Krowiak <[email protected]>
> Reviewed-by: Cédric Le Goater <[email protected]>
FYI, this patch (or some part of this series) breaks hotplug for PCI devices on
s390x. I verified it breaks with both PCI passthrough and virtio-pci via virsh
attach-device.
I get greeted with the following message:
qemu-system-s390x: ../util/qemu-thread-posix.c:92: qemu_mutex_lock_impl:
Assertion `mutex->initialized' failed.
2025-06-26 14:12:23.031+0000: shutting down, reason=crashed
As a quick test, reverting this patch resolves the issue.
Another test shows that if I have a vfio-ap device attached to my guest before
I try the zPCI hotplug, everything works fine (because vfio_ap_realize was
driven -- see more info below).
I note in the failing call chain that in response to the hotplug event for a
zPCI device we end up calling into hw/vfio/ap.c and getting some locks which
seems wrong:
(gdb) bt
#0 __pthread_kill_implementation (threadid=<optimized out>,
signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
#1 0x000003ffa85ad636 in __pthread_kill_internal (threadid=<optimized out>,
signo=6) at pthread_kill.c:78
#2 0x000003ffa8553c20 in __GI_raise (sig=sig@entry=6) at
../sysdeps/posix/raise.c:26
#3 0x000003ffa8533e3c in __GI_abort () at abort.c:79
#4 0x000003ffa854ac9a in __assert_fail_base
(fmt=<optimized out>, assertion=assertion@entry=0x2aa3fa76414
"mutex->initialized", file=file@entry=0x2aa3fa763f8
"../util/qemu-thread-posix.c", line=line@entry=92,
function=function@entry=0x2aa3fa7647a <__PRETTY_FUNCTION__.23>
"qemu_mutex_lock_impl") at assert.c:94
#5 0x000003ffa854acf4 in __assert_fail
(assertion=0x2aa3fa76414 "mutex->initialized", file=0x2aa3fa763f8
"../util/qemu-thread-posix.c", line=92, function=0x2aa3fa7647a
<__PRETTY_FUNCTION__.23> "qemu_mutex_lock_impl") at assert.c:103
#6 0x000002aa3f83ef02 in qemu_mutex_lock_impl (mutex=0x2aa3fed9e08
<cfg_chg_events_lock>, file=0x2aa3f9e653e
"/usr/src/qemu/include/qemu/lockable.h", line=56) at
../util/qemu-thread-posix.c:92
#7 0x000002aa3f4c3a70 in qemu_lockable_mutex_lock (x=0x2aa3fed9e08
<cfg_chg_events_lock>) at /usr/src/qemu/include/qemu/lockable.h:56
#8 0x000002aa3f4c3b00 in qemu_lockable_lock (x=0x3fd076f7730) at
/usr/src/qemu/include/qemu/lockable.h:110
#9 0x000002aa3f4c3b82 in qemu_lockable_auto_lock (x=0x3fd076f7730) at
/usr/src/qemu/include/qemu/lockable.h:120
#10 0x000002aa3f4c3f90 in ap_chsc_sei_nt0_get_event (res=0x3fd076f79b8) at
../hw/vfio/ap.c:110
^^^^^^^^
#11 0x000002aa3f3a59ea in chsc_sei_nt0_get_event (res=0x3fd076f79b8) at
../target/s390x/ioinst.c:579
#12 0x000002aa3f3a5b76 in ioinst_handle_chsc_sei (req=0x3fd076f79a8,
res=0x3fd076f79b8) at ../target/s390x/ioinst.c:620
#13 0x000002aa3f3a6062 in ioinst_handle_chsc (cpu=0x2aa40cc9430, ipb=2097152,
ra=0) at ../target/s390x/ioinst.c:708
#14 0x000002aa3f4647d2 in handle_b2 (cpu=0x2aa40cc9430, run=0x3ffa4180000,
ipa1=95 '_') at ../target/s390x/kvm/kvm.c:1218
#15 0x000002aa3f465db2 in handle_instruction (cpu=0x2aa40cc9430,
run=0x3ffa4180000) at ../target/s390x/kvm/kvm.c:1627
#16 0x000002aa3f4661da in handle_intercept (cpu=0x2aa40cc9430) at
../target/s390x/kvm/kvm.c:1709
#17 0x000002aa3f466c10 in kvm_arch_handle_exit (cs=0x2aa40cc9430,
run=0x3ffa4180000) at ../target/s390x/kvm/kvm.c:1914
#18 0x000002aa3f523c98 in kvm_cpu_exec (cpu=0x2aa40cc9430) at
../accel/kvm/kvm-all.c:3294
#19 0x000002aa3f527f52 in kvm_vcpu_thread_fn (arg=0x2aa40cc9430) at
../accel/kvm/kvm-accel-ops.c:51
#20 0x000002aa3f840002 in qemu_thread_start (args=0x2aa40ccd090) at
../util/qemu-thread-posix.c:393
#21 0x000003ffa85ab622 in start_thread (arg=0x3fd076fe8c0) at
pthread_create.c:448
#22 0x000003ffa862b390 in thread_start () at
../sysdeps/unix/sysv/linux/s390/s390-64/clone3.S:71
Note that PCI devices were already using SEI before this series (e.g. see
hw/s390x/s390-pci-bus.c:pci_chsc_sei_nt2_get_event()); I think the problem here
is that nt0 events are looked for implicitly in the shared handler
(ioinst_handle_chsc_sei()) which is fine -- but you cannot go acquiring locks
from ap.c that might have never been initialized (cfg_chg_events_lock which is
only initialized in vfio_ap_realize()); that's why we ultimately crash.
AFAICT this portion of the handler including mutex should be moved out of ap.c
into chsc_sei_nt0_get_event(). When vfio_ap is not in use, we can't call into
it. Can't you also build without VFIO_AP? I didn't try it but that sure seems
like it would cause issues too.
I guess the mutex and list would then need to be part of something common to
s390x like S390CcwMachineState.
Alternatively, you could leave those in vfio_ap but would need some kind of
indicator that vfio_ap is "armed" for these kinds of events and only bother
calling into vfio_ap to look at cfg_chg_events when that indicator has been set
(e.g. vfio_ap_realize has been called once. And it actually looks like you
already do something like this via lock_initialized in ap.c).
Otherwise, we should always return EVENT_INFORMATION_NOT_STORED like the old
code did (via return 1).
Or maybe someone else has a better idea.
> ---
> 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 <[email protected]>
> + *
> + * 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;
> }
>
