[RFC PATCH] hw/s390x: add vhost-user-device-ccw stub (!WIP)

[RFC PATCH] hw/s390x: add vhost-user-device-ccw stub (!WIP)

[Alex Bennée]

This Friday hack was driven by wanting to review some virtiofsd
changes. However I discovered that while virtiofsd support
VHOST_USER_PROTOCOL_F_CONFIG the current virtiofs stubs still assume
QEMU controls the config space.

Ahh I thought, but we have vhost-user-base which both supports this
and provides for a very minimal boilerplate implementation of the
various VirtIO devices. However s390x being a special beast doesn't
use mmio or pci transports but has its own special ccw transport.
Perhaps I could just follow the existing ccw devices and implement the
wrapper. So here we are:

  ./qemu-system-s390x -serial mon:stdio \
      -M s390-ccw-virtio -m 2048M -smp 2 \
      -drive file=/home/alex/lsrc/tests/buildroot.git/builds/s390x/images/rootfs.ext4,if=virtio,format=raw -snapshot \
      -kernel /home/alex/lsrc/linux.git/builds/s390x/arch/s390/boot/bzImage \
      -append "root=/dev/vda console=ttyS0" -display none \
      -chardev socket,id=vfs,path=/tmp/virtiofsd.sock \
      -device vhost-user-device-ccw,chardev=vfs,virtio-id=26,config_size=44,num_vqs=1 -s -S

with:

  ./target/debug/virtiofsd --shared-dir $HOME \
    --socket-path /tmp/virtiofsd.sock --tag home --log-level debug

and the kernel gets surprisingly far before oopsing:

  [    2.084873] virtio_blk virtio1: 1/0/0 default/read/poll queues
  [    2.097845] virtiofs virtio2: discovered new tag: home

heh I think GET_CONFIG worked because the tag can only have come from
the daemon.

  [    2.101920] Segment Routing with IPv6
  [    2.102040] RPL Segment Routing with IPv6
  [    2.102326] In-situ OAM (IOAM) with IPv6
  [    2.103443] NET: Registered PF_PACKET protocol family
  [    2.103848] virtio_blk virtio1: [vda] 122880 512-byte logical blocks (62.9 MB/60.0 MiB)
  [    2.104918] Key type dns_resolver registered
  [    2.106171] virtiofs virtio2: probe with driver virtiofs failed with error -2

Hmm disappointing...

  [    2.113538] Unable to handle kernel pointer dereference in virtual kernel address space
  [    2.113650] Failing address: 766461000242c000 TEID: 766461000242c803
  [    2.113697] Fault in home space mode while using kernel ASCE.
  [    2.113881] AS:0000000001e08007 R3:0000000000000024
  [    2.114679] Oops: 0038 ilc:3 [#1] SMP
  [    2.114837] Modules linked in:
  [    2.115351] CPU: 1 UID: 0 PID: 30 Comm: kworker/u10:1 Not tainted 6.14.5-ajb #8
  [    2.115629] Hardware name: QEMU 8561 QEMU (KVM/Linux)
  [    2.115850] Workqueue: async async_run_entry_fn

At which point my attempts to debug the s390 kernel failed because the
debug symbols don't seem to match where the kernel lives. Also its
Friday and I have a cold beer waiting for me in the fridge.

So it seems frustratingly close to being functional but as I'm going
to be away for a week I thought it was worth posting as an RFC in case
anyone wants to take a closer look.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 hw/s390x/vhost-user-device-ccw.c | 75 ++++++++++++++++++++++++++++++++
 hw/s390x/meson.build             | 18 +++++---
 2 files changed, 88 insertions(+), 5 deletions(-)
 create mode 100644 hw/s390x/vhost-user-device-ccw.c

diff --git a/hw/s390x/vhost-user-device-ccw.c b/hw/s390x/vhost-user-device-ccw.c
new file mode 100644
index 0000000000..03881efc02
--- /dev/null
+++ b/hw/s390x/vhost-user-device-ccw.c
@@ -0,0 +1,75 @@
+/*
+ * Vhost-user generic device virtio-ccw glue
+ *
+ * Copyright (c) 2025 Linaro Ltd.
+ * Author: Alex Bennée <alex.bennee@linaro.org>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "hw/qdev-properties.h"
+#include "hw/virtio/vhost-user-base.h"
+#include "virtio-ccw.h"
+
+struct VHostUserDeviceCCW {
+    VirtioCcwDevice parent_obj;
+
+    VHostUserBase vub;
+};
+
+static const Property vhost_user_ccw_properties[] = {
+    DEFINE_PROP_BIT("ioeventfd", VirtioCcwDevice, flags,
+                    VIRTIO_CCW_FLAG_USE_IOEVENTFD_BIT, true),
+    DEFINE_PROP_UINT32("max_revision", VirtioCcwDevice, max_rev,
+                       VIRTIO_CCW_MAX_REV),
+};
+
+#define TYPE_VHOST_USER_DEVICE_CCW "vhost-user-device-ccw"
+
+OBJECT_DECLARE_SIMPLE_TYPE(VHostUserDeviceCCW, VHOST_USER_DEVICE_CCW)
+
+static void vhost_user_device_ccw_realize(VirtioCcwDevice *ccw_dev, Error **errp)
+{
+    VHostUserDeviceCCW *dev = VHOST_USER_DEVICE_CCW(ccw_dev);
+    DeviceState *vdev = DEVICE(&dev->vub);
+
+    qdev_realize(vdev, BUS(&ccw_dev->bus), errp);
+}
+
+static void vhost_user_device_ccw_instance_init(Object *obj)
+{
+    VHostUserDeviceCCW *dev = VHOST_USER_DEVICE_CCW(obj);
+    VirtioCcwDevice *ccw_dev = VIRTIO_CCW_DEVICE(obj);
+
+    ccw_dev->force_revision_1 = true;
+    virtio_instance_init_common(obj, &dev->vub, sizeof(dev->vub),
+                                TYPE_VHOST_USER_DEVICE);
+}
+
+static void vhost_user_device_ccw_class_init(ObjectClass *klass, const void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+    VirtIOCCWDeviceClass *k = VIRTIO_CCW_DEVICE_CLASS(klass);
+
+    /* Reason: stop users confusing themselves */
+    /* dc->user_creatable = false; */
+
+    k->realize = vhost_user_device_ccw_realize;
+    device_class_set_props(dc, vhost_user_ccw_properties);
+}
+
+static const TypeInfo vhost_user_device_ccw = {
+    .name          = TYPE_VHOST_USER_DEVICE_CCW,
+    .parent        = TYPE_VIRTIO_CCW_DEVICE,
+    .instance_size = sizeof(VHostUserDeviceCCW),
+    .instance_init = vhost_user_device_ccw_instance_init,
+    .class_init    = vhost_user_device_ccw_class_init,
+};
+
+static void vhost_user_device_ccw_register(void)
+{
+    type_register_static(&vhost_user_device_ccw);
+}
+
+type_init(vhost_user_device_ccw_register)
diff --git a/hw/s390x/meson.build b/hw/s390x/meson.build
index 3bbebfd817..a311ccca8d 100644
--- a/hw/s390x/meson.build
+++ b/hw/s390x/meson.build
@@ -44,14 +44,22 @@ virtio_ss.add(when: 'CONFIG_VIRTIO_NET', if_true: files('virtio-ccw-net.c'))
 virtio_ss.add(when: 'CONFIG_VIRTIO_RNG', if_true: files('virtio-ccw-rng.c'))
 virtio_ss.add(when: 'CONFIG_VIRTIO_SCSI', if_true: files('virtio-ccw-scsi.c'))
 virtio_ss.add(when: 'CONFIG_VIRTIO_SERIAL', if_true: files('virtio-ccw-serial.c'))
+virtio_ss.add(when: 'CONFIG_VIRTIO_MD', if_true: files('virtio-ccw-md.c'))
+virtio_ss.add(when: 'CONFIG_VIRTIO_MEM', if_true: files('virtio-ccw-mem.c'))
 if have_virtfs
   virtio_ss.add(when: 'CONFIG_VIRTIO_9P', if_true: files('virtio-ccw-9p.c'))
 endif
-virtio_ss.add(when: 'CONFIG_VHOST_SCSI', if_true: files('vhost-scsi-ccw.c'))
-virtio_ss.add(when: 'CONFIG_VHOST_VSOCK', if_true: files('vhost-vsock-ccw.c'))
-virtio_ss.add(when: 'CONFIG_VHOST_USER_FS', if_true: files('vhost-user-fs-ccw.c'))
-virtio_ss.add(when: 'CONFIG_VIRTIO_MD', if_true: files('virtio-ccw-md.c'))
-virtio_ss.add(when: 'CONFIG_VIRTIO_MEM', if_true: files('virtio-ccw-mem.c'))
+
+if have_vhost
+  virtio_ss.add(when: 'CONFIG_VHOST_SCSI', if_true: files('vhost-scsi-ccw.c'))
+  virtio_ss.add(when: 'CONFIG_VHOST_VSOCK', if_true: files('vhost-vsock-ccw.c'))
+
+  if have_vhost_user
+    virtio_ss.add(files('vhost-user-device-ccw.c'))
+    virtio_ss.add(when: 'CONFIG_VHOST_USER_FS', if_true: files('vhost-user-fs-ccw.c'))
+  endif
+endif
+
 s390x_ss.add_all(when: 'CONFIG_VIRTIO_CCW', if_true: virtio_ss)
 
 s390x_ss.add(when: 'CONFIG_VIRTIO_MD', if_false: files('virtio-ccw-md-stubs.c'))
-- 
2.39.5