hw/xen: Add xen_evtchn device for event channel emulation

Include basic support for setting HVM_PARAM_CALLBACK_IRQ to the global
vector method HVM_PARAM_CALLBACK_TYPE_VECTOR, which is handled in-kernel
by raising the vector whenever the vCPU's vcpu_info->evtchn_upcall_pending
flag is set.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

hw/i386/kvm/meson.build

@@ -4,6 +4,9 @@ i386_kvm_ss.add(when: 'CONFIG_APIC', if_true: files('apic.c'))
 i386_kvm_ss.add(when: 'CONFIG_I8254', if_true: files('i8254.c'))
 i386_kvm_ss.add(when: 'CONFIG_I8259', if_true: files('i8259.c'))
 i386_kvm_ss.add(when: 'CONFIG_IOAPIC', if_true: files('ioapic.c'))
-i386_kvm_ss.add(when: 'CONFIG_XEN_EMU', if_true: files('xen_overlay.c'))
+i386_kvm_ss.add(when: 'CONFIG_XEN_EMU', if_true: files(
+  'xen_overlay.c',
+  'xen_evtchn.c',
+  ))
 
 i386_ss.add_all(when: 'CONFIG_KVM', if_true: i386_kvm_ss)

hw/i386/kvm/xen_evtchn.c

@@ -0,0 +1,155 @@
+/*
+ * QEMU Xen emulation: Event channel support
+ *
+ * Copyright © 2022 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Authors: David Woodhouse <dwmw2@infradead.org>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/host-utils.h"
+#include "qemu/module.h"
+#include "qemu/main-loop.h"
+#include "qapi/error.h"
+#include "qom/object.h"
+#include "exec/target_page.h"
+#include "exec/address-spaces.h"
+#include "migration/vmstate.h"
+
+#include "hw/sysbus.h"
+#include "hw/xen/xen.h"
+#include "xen_evtchn.h"
+
+#include "sysemu/kvm.h"
+#include "sysemu/kvm_xen.h"
+#include <linux/kvm.h>
+
+#include "hw/xen/interface/memory.h"
+#include "hw/xen/interface/hvm/params.h"
+
+#define TYPE_XEN_EVTCHN "xen-evtchn"
+OBJECT_DECLARE_SIMPLE_TYPE(XenEvtchnState, XEN_EVTCHN)
+
+struct XenEvtchnState {
+    /*< private >*/
+    SysBusDevice busdev;
+    /*< public >*/
+
+    uint64_t callback_param;
+    bool evtchn_in_kernel;
+
+    QemuMutex port_lock;
+};
+
+struct XenEvtchnState *xen_evtchn_singleton;
+
+/* Top bits of callback_param are the type (HVM_PARAM_CALLBACK_TYPE_xxx) */
+#define CALLBACK_VIA_TYPE_SHIFT 56
+
+static int xen_evtchn_post_load(void *opaque, int version_id)
+{
+    XenEvtchnState *s = opaque;
+
+    if (s->callback_param) {
+        xen_evtchn_set_callback_param(s->callback_param);
+    }
+
+    return 0;
+}
+
+static bool xen_evtchn_is_needed(void *opaque)
+{
+    return xen_mode == XEN_EMULATE;
+}
+
+static const VMStateDescription xen_evtchn_vmstate = {
+    .name = "xen_evtchn",
+    .version_id = 1,
+    .minimum_version_id = 1,
+    .needed = xen_evtchn_is_needed,
+    .post_load = xen_evtchn_post_load,
+    .fields = (VMStateField[]) {
+        VMSTATE_UINT64(callback_param, XenEvtchnState),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
+static void xen_evtchn_class_init(ObjectClass *klass, void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+
+    dc->vmsd = &xen_evtchn_vmstate;
+}
+
+static const TypeInfo xen_evtchn_info = {
+    .name          = TYPE_XEN_EVTCHN,
+    .parent        = TYPE_SYS_BUS_DEVICE,
+    .instance_size = sizeof(XenEvtchnState),
+    .class_init    = xen_evtchn_class_init,
+};
+
+void xen_evtchn_create(void)
+{
+    XenEvtchnState *s = XEN_EVTCHN(sysbus_create_simple(TYPE_XEN_EVTCHN,
+                                                        -1, NULL));
+    xen_evtchn_singleton = s;
+
+    qemu_mutex_init(&s->port_lock);
+}
+
+static void xen_evtchn_register_types(void)
+{
+    type_register_static(&xen_evtchn_info);
+}
+
+type_init(xen_evtchn_register_types)
+
+int xen_evtchn_set_callback_param(uint64_t param)
+{
+    XenEvtchnState *s = xen_evtchn_singleton;
+    struct kvm_xen_hvm_attr xa = {
+        .type = KVM_XEN_ATTR_TYPE_UPCALL_VECTOR,
+        .u.vector = 0,
+    };
+    bool in_kernel = false;
+    int ret;
+
+    if (!s) {
+        return -ENOTSUP;
+    }
+
+    qemu_mutex_lock(&s->port_lock);
+
+    switch (param >> CALLBACK_VIA_TYPE_SHIFT) {
+    case HVM_PARAM_CALLBACK_TYPE_VECTOR: {
+        xa.u.vector = (uint8_t)param,
+
+        ret = kvm_vm_ioctl(kvm_state, KVM_XEN_HVM_SET_ATTR, &xa);
+        if (!ret && kvm_xen_has_cap(EVTCHN_SEND)) {
+            in_kernel = true;
+        }
+        break;
+    }
+    default:
+        /* Xen doesn't return error even if you set something bogus */
+        ret = 0;
+        break;
+    }
+
+    if (!ret) {
+        /* If vector delivery was turned *off* then tell the kernel */
+        if ((s->callback_param >> CALLBACK_VIA_TYPE_SHIFT) ==
+            HVM_PARAM_CALLBACK_TYPE_VECTOR && !xa.u.vector) {
+            kvm_vm_ioctl(kvm_state, KVM_XEN_HVM_SET_ATTR, &xa);
+        }
+        s->callback_param = param;
+        s->evtchn_in_kernel = in_kernel;
+    }
+
+    qemu_mutex_unlock(&s->port_lock);
+
+    return ret;
+}

hw/i386/kvm/xen_evtchn.h

@@ -0,0 +1,18 @@
+/*
+ * QEMU Xen emulation: Event channel support
+ *
+ * Copyright © 2022 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Authors: David Woodhouse <dwmw2@infradead.org>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#ifndef QEMU_XEN_EVTCHN_H
+#define QEMU_XEN_EVTCHN_H
+
+void xen_evtchn_create(void);
+int xen_evtchn_set_callback_param(uint64_t param);
+
+#endif /* QEMU_XEN_EVTCHN_H */

hw/i386/pc.c

@@ -91,6 +91,7 @@
 #include "hw/virtio/virtio-pmem-pci.h"
 #include "hw/virtio/virtio-mem-pci.h"
 #include "hw/i386/kvm/xen_overlay.h"
+#include "hw/i386/kvm/xen_evtchn.h"
 #include "hw/mem/memory-device.h"
 #include "sysemu/replay.h"
 #include "target/i386/cpu.h"
@@ -1852,6 +1853,7 @@ int pc_machine_kvm_type(MachineState *machine, const char *kvm_type)
 #ifdef CONFIG_XEN_EMU
     if (xen_mode == XEN_EMULATE) {
         xen_overlay_create();
+        xen_evtchn_create();
     }
 #endif
     return 0;

target/i386/kvm/xen-emu.c

@@ -22,6 +22,7 @@
 #include "sysemu/runstate.h"
 
 #include "hw/i386/kvm/xen_overlay.h"
+#include "hw/i386/kvm/xen_evtchn.h"
 
 #include "hw/xen/interface/version.h"
 #include "hw/xen/interface/sched.h"
@@ -510,6 +511,10 @@ static bool handle_set_param(struct kvm_xen_exit *exit, X86CPU *cpu,
     }
 
     switch (hp.index) {
+    case HVM_PARAM_CALLBACK_IRQ:
+        err = xen_evtchn_set_callback_param(hp.value);
+        xen_set_long_mode(exit->u.hcall.longmode);
+        break;
     default:
         return false;
     }
@@ -719,6 +724,16 @@ int kvm_xen_soft_reset(void)
 
     trace_kvm_xen_soft_reset();
 
+    /*
+     * Zero is the reset/startup state for HVM_PARAM_CALLBACK_IRQ. Strictly,
+     * it maps to HVM_PARAM_CALLBACK_TYPE_GSI with GSI#0, but Xen refuses to
+     * to deliver to the timer interrupt and treats that as 'disabled'.
+     */
+    err = xen_evtchn_set_callback_param(0);
+    if (err) {
+        return err;
+    }
+
     CPU_FOREACH(cpu) {
         async_run_on_cpu(cpu, do_vcpu_soft_reset, RUN_ON_CPU_NULL);
     }