Move static event channel feature related code to a separate module
(static-evtchn.{c,h}) in the spirit of fine granular configuration, so
that the feature can be disabled if not needed.

Introduce Kconfig option CONFIG_STATIC_EVTCHN, enabled by default (to
keep the current behavior) dependent on CONFIG_DOM0LESS. While it could
be possible to create a loopback connection for dom0 only, this use case
does not really need this feature and all the docs and commit messages
refer explicitly to the use in dom0less system.

The only function visible externally is alloc_static_evtchn(), so move
the prototype to static-evtchn.h and provide a stub in case a feature
is disabled. Guard static_evtchn_created in struct dt_device_node and
move its helpers to static-evtchn.h.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
---
This is a follow up to Luca's dom0less features modularization.
I'm not sure what's the best strategy for static_evtchn_created helpers.
We could also guard them in device_tree.h for ease of removal when we find
a better way for handling the flag.
---
 xen/arch/arm/Kconfig                     |   8 ++
 xen/arch/arm/Makefile                    |   1 +
 xen/arch/arm/domain_build.c              | 146 ---------------------
 xen/arch/arm/include/asm/setup.h         |   1 -
 xen/arch/arm/include/asm/static-evtchn.h |  41 ++++++
 xen/arch/arm/setup.c                     |   1 +
 xen/arch/arm/static-evtchn.c             | 159 +++++++++++++++++++++++
 xen/include/xen/device_tree.h            |  14 +-
 8 files changed, 212 insertions(+), 159 deletions(-)
 create mode 100644 xen/arch/arm/include/asm/static-evtchn.h
 create mode 100644 xen/arch/arm/static-evtchn.c

diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
index f73b62e50d00..50e9bfae1ac8 100644
--- a/xen/arch/arm/Kconfig
+++ b/xen/arch/arm/Kconfig
@@ -217,6 +217,14 @@ config STATIC_SHM
 	help
 	  This option enables statically shared memory on a dom0less system.
 
+config STATIC_EVTCHN
+	bool "Static event channel support on a dom0less system"
+	depends on DOM0LESS_BOOT
+	default y
+	help
+	  This option enables establishing static event channel communication
+	  between domains on a dom0less system (domU-domU as well as domU-dom0).
+
 endmenu
 
 menu "ARM errata workaround via the alternative framework"
diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
index 809772417c14..33c677672fe6 100644
--- a/xen/arch/arm/Makefile
+++ b/xen/arch/arm/Makefile
@@ -51,6 +51,7 @@ obj-y += setup.o
 obj-y += shutdown.o
 obj-y += smp.o
 obj-y += smpboot.o
+obj-$(CONFIG_STATIC_EVTCHN) += static-evtchn.init.o
 obj-$(CONFIG_STATIC_MEMORY) += static-memory.init.o
 obj-$(CONFIG_STATIC_SHM) += static-shmem.init.o
 obj-y += sysctl.o
diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index df66fb88d8ec..613b2885cebb 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -38,8 +38,6 @@
 #include <xen/grant_table.h>
 #include <xen/serial.h>
 
-#define STATIC_EVTCHN_NODE_SIZE_CELLS 2
-
 static unsigned int __initdata opt_dom0_max_vcpus;
 integer_param("dom0_max_vcpus", opt_dom0_max_vcpus);
 
@@ -1919,150 +1917,6 @@ void __init evtchn_allocate(struct domain *d)
     d->arch.hvm.params[HVM_PARAM_CALLBACK_IRQ] = val;
 }
 
-static int __init get_evtchn_dt_property(const struct dt_device_node *np,
-                                         uint32_t *port, uint32_t *phandle)
-{
-    const __be32 *prop = NULL;
-    uint32_t len;
-
-    prop = dt_get_property(np, "xen,evtchn", &len);
-    if ( !prop )
-    {
-        printk(XENLOG_ERR "xen,evtchn property should not be empty.\n");
-        return -EINVAL;
-    }
-
-    if ( !len || len < dt_cells_to_size(STATIC_EVTCHN_NODE_SIZE_CELLS) )
-    {
-        printk(XENLOG_ERR "xen,evtchn property value is not valid.\n");
-        return -EINVAL;
-    }
-
-    *port = dt_next_cell(1, &prop);
-    *phandle = dt_next_cell(1, &prop);
-
-    return 0;
-}
-
-static int __init alloc_domain_evtchn(struct dt_device_node *node)
-{
-    int rc;
-    uint32_t domU1_port, domU2_port, remote_phandle;
-    struct dt_device_node *remote_node;
-    const struct dt_device_node *p1_node, *p2_node;
-    struct evtchn_alloc_unbound alloc_unbound;
-    struct evtchn_bind_interdomain bind_interdomain;
-    struct domain *d1 = NULL, *d2 = NULL;
-
-    if ( !dt_device_is_compatible(node, "xen,evtchn-v1") )
-        return 0;
-
-    /*
-     * Event channel is already created while parsing the other side of
-     * evtchn node.
-     */
-    if ( dt_device_static_evtchn_created(node) )
-        return 0;
-
-    rc = get_evtchn_dt_property(node, &domU1_port, &remote_phandle);
-    if ( rc )
-        return rc;
-
-    remote_node = dt_find_node_by_phandle(remote_phandle);
-    if ( !remote_node )
-    {
-        printk(XENLOG_ERR
-                "evtchn: could not find remote evtchn phandle\n");
-        return -EINVAL;
-    }
-
-    rc = get_evtchn_dt_property(remote_node, &domU2_port, &remote_phandle);
-    if ( rc )
-        return rc;
-
-    if ( node->phandle != remote_phandle )
-    {
-        printk(XENLOG_ERR "xen,evtchn property is not setup correctly.\n");
-        return -EINVAL;
-    }
-
-    p1_node = dt_get_parent(node);
-    if ( !p1_node )
-    {
-        printk(XENLOG_ERR "evtchn: evtchn parent node is NULL\n" );
-        return -EINVAL;
-    }
-
-    p2_node = dt_get_parent(remote_node);
-    if ( !p2_node )
-    {
-        printk(XENLOG_ERR "evtchn: remote parent node is NULL\n" );
-        return -EINVAL;
-    }
-
-    d1 = get_domain_by_id(p1_node->used_by);
-    d2 = get_domain_by_id(p2_node->used_by);
-
-    if ( !d1 || !d2 )
-    {
-        printk(XENLOG_ERR "evtchn: could not find domains\n" );
-        return -EINVAL;
-    }
-
-    alloc_unbound.dom = d1->domain_id;
-    alloc_unbound.remote_dom = d2->domain_id;
-
-    rc = evtchn_alloc_unbound(&alloc_unbound, domU1_port);
-    if ( rc < 0 )
-    {
-        printk(XENLOG_ERR
-                "evtchn_alloc_unbound() failure (Error %d) \n", rc);
-        return rc;
-    }
-
-    bind_interdomain.remote_dom  = d1->domain_id;
-    bind_interdomain.remote_port = domU1_port;
-
-    rc = evtchn_bind_interdomain(&bind_interdomain, d2, domU2_port);
-    if ( rc < 0 )
-    {
-        printk(XENLOG_ERR
-                "evtchn_bind_interdomain() failure (Error %d) \n", rc);
-        return rc;
-    }
-
-    dt_device_set_static_evtchn_created(node);
-    dt_device_set_static_evtchn_created(remote_node);
-
-    return 0;
-}
-
-void __init alloc_static_evtchn(void)
-{
-    struct dt_device_node *node, *evtchn_node;
-    struct dt_device_node *chosen = dt_find_node_by_path("/chosen");
-
-    BUG_ON(chosen == NULL);
-
-    if ( hardware_domain )
-        dt_device_set_used_by(chosen, hardware_domain->domain_id);
-
-    dt_for_each_child_node(chosen, node)
-    {
-        if ( hardware_domain )
-        {
-            if ( alloc_domain_evtchn(node) != 0 )
-                panic("Could not set up domains evtchn\n");
-        }
-
-        dt_for_each_child_node(node, evtchn_node)
-        {
-            if ( alloc_domain_evtchn(evtchn_node) != 0 )
-                panic("Could not set up domains evtchn\n");
-        }
-    }
-}
-
 static void __init find_gnttab_region(struct domain *d,
                                       struct kernel_info *kinfo)
 {
diff --git a/xen/arch/arm/include/asm/setup.h b/xen/arch/arm/include/asm/setup.h
index bda3c07b8797..d15a88d2e0d1 100644
--- a/xen/arch/arm/include/asm/setup.h
+++ b/xen/arch/arm/include/asm/setup.h
@@ -136,7 +136,6 @@ void acpi_create_efi_mmap_table(struct domain *d,
 int acpi_make_efi_nodes(void *fdt, struct membank tbl_add[]);
 
 void create_dom0(void);
-void alloc_static_evtchn(void);
 
 void discard_initial_modules(void);
 void fw_unreserved_regions(paddr_t s, paddr_t e,
diff --git a/xen/arch/arm/include/asm/static-evtchn.h b/xen/arch/arm/include/asm/static-evtchn.h
new file mode 100644
index 000000000000..472673fae345
--- /dev/null
+++ b/xen/arch/arm/include/asm/static-evtchn.h
@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef __ASM_STATIC_EVTCHN_H_
+#define __ASM_STATIC_EVTCHN_H_
+
+#ifdef CONFIG_STATIC_EVTCHN
+
+#include <xen/device_tree.h>
+
+#define STATIC_EVTCHN_NODE_SIZE_CELLS 2
+
+void alloc_static_evtchn(void);
+
+static inline void
+dt_device_set_static_evtchn_created(struct dt_device_node *device)
+{
+    device->static_evtchn_created = true;
+}
+
+static inline bool
+dt_device_static_evtchn_created(const struct dt_device_node *device)
+{
+    return device->static_evtchn_created;
+}
+
+#else /* !CONFIG_STATIC_EVTCHN */
+
+static inline void alloc_static_evtchn(void) {};
+
+#endif /* CONFIG_STATIC_EVTCHN */
+
+#endif /* __ASM_STATIC_EVTCHN_H_ */
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index d07ea044db48..59dd9bb25a9f 100644
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -36,6 +36,7 @@
 #include <asm/alternative.h>
 #include <asm/dom0less-build.h>
 #include <asm/page.h>
+#include <asm/static-evtchn.h>
 #include <asm/current.h>
 #include <asm/setup.h>
 #include <asm/gic.h>
diff --git a/xen/arch/arm/static-evtchn.c b/xen/arch/arm/static-evtchn.c
new file mode 100644
index 000000000000..b824694762ec
--- /dev/null
+++ b/xen/arch/arm/static-evtchn.c
@@ -0,0 +1,159 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <xen/event.h>
+
+#include <asm/static-evtchn.h>
+
+static int __init get_evtchn_dt_property(const struct dt_device_node *np,
+                                         uint32_t *port, uint32_t *phandle)
+{
+    const __be32 *prop = NULL;
+    uint32_t len;
+
+    prop = dt_get_property(np, "xen,evtchn", &len);
+    if ( !prop )
+    {
+        printk(XENLOG_ERR "xen,evtchn property should not be empty.\n");
+        return -EINVAL;
+    }
+
+    if ( !len || len < dt_cells_to_size(STATIC_EVTCHN_NODE_SIZE_CELLS) )
+    {
+        printk(XENLOG_ERR "xen,evtchn property value is not valid.\n");
+        return -EINVAL;
+    }
+
+    *port = dt_next_cell(1, &prop);
+    *phandle = dt_next_cell(1, &prop);
+
+    return 0;
+}
+
+static int __init alloc_domain_evtchn(struct dt_device_node *node)
+{
+    int rc;
+    uint32_t domU1_port, domU2_port, remote_phandle;
+    struct dt_device_node *remote_node;
+    const struct dt_device_node *p1_node, *p2_node;
+    struct evtchn_alloc_unbound alloc_unbound;
+    struct evtchn_bind_interdomain bind_interdomain;
+    struct domain *d1 = NULL, *d2 = NULL;
+
+    if ( !dt_device_is_compatible(node, "xen,evtchn-v1") )
+        return 0;
+
+    /*
+     * Event channel is already created while parsing the other side of
+     * evtchn node.
+     */
+    if ( dt_device_static_evtchn_created(node) )
+        return 0;
+
+    rc = get_evtchn_dt_property(node, &domU1_port, &remote_phandle);
+    if ( rc )
+        return rc;
+
+    remote_node = dt_find_node_by_phandle(remote_phandle);
+    if ( !remote_node )
+    {
+        printk(XENLOG_ERR
+                "evtchn: could not find remote evtchn phandle\n");
+        return -EINVAL;
+    }
+
+    rc = get_evtchn_dt_property(remote_node, &domU2_port, &remote_phandle);
+    if ( rc )
+        return rc;
+
+    if ( node->phandle != remote_phandle )
+    {
+        printk(XENLOG_ERR "xen,evtchn property is not setup correctly.\n");
+        return -EINVAL;
+    }
+
+    p1_node = dt_get_parent(node);
+    if ( !p1_node )
+    {
+        printk(XENLOG_ERR "evtchn: evtchn parent node is NULL\n" );
+        return -EINVAL;
+    }
+
+    p2_node = dt_get_parent(remote_node);
+    if ( !p2_node )
+    {
+        printk(XENLOG_ERR "evtchn: remote parent node is NULL\n" );
+        return -EINVAL;
+    }
+
+    d1 = get_domain_by_id(p1_node->used_by);
+    d2 = get_domain_by_id(p2_node->used_by);
+
+    if ( !d1 || !d2 )
+    {
+        printk(XENLOG_ERR "evtchn: could not find domains\n" );
+        return -EINVAL;
+    }
+
+    alloc_unbound.dom = d1->domain_id;
+    alloc_unbound.remote_dom = d2->domain_id;
+
+    rc = evtchn_alloc_unbound(&alloc_unbound, domU1_port);
+    if ( rc < 0 )
+    {
+        printk(XENLOG_ERR
+                "evtchn_alloc_unbound() failure (Error %d) \n", rc);
+        return rc;
+    }
+
+    bind_interdomain.remote_dom  = d1->domain_id;
+    bind_interdomain.remote_port = domU1_port;
+
+    rc = evtchn_bind_interdomain(&bind_interdomain, d2, domU2_port);
+    if ( rc < 0 )
+    {
+        printk(XENLOG_ERR
+                "evtchn_bind_interdomain() failure (Error %d) \n", rc);
+        return rc;
+    }
+
+    dt_device_set_static_evtchn_created(node);
+    dt_device_set_static_evtchn_created(remote_node);
+
+    return 0;
+}
+
+void __init alloc_static_evtchn(void)
+{
+    struct dt_device_node *node, *evtchn_node;
+    struct dt_device_node *chosen = dt_find_node_by_path("/chosen");
+
+    BUG_ON(chosen == NULL);
+
+    if ( hardware_domain )
+        dt_device_set_used_by(chosen, hardware_domain->domain_id);
+
+    dt_for_each_child_node(chosen, node)
+    {
+        if ( hardware_domain )
+        {
+            if ( alloc_domain_evtchn(node) != 0 )
+                panic("Could not set up domains evtchn\n");
+        }
+
+        dt_for_each_child_node(node, evtchn_node)
+        {
+            if ( alloc_domain_evtchn(evtchn_node) != 0 )
+                panic("Could not set up domains evtchn\n");
+        }
+    }
+}
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/include/xen/device_tree.h b/xen/include/xen/device_tree.h
index 3ae7b45429b6..492363bcde10 100644
--- a/xen/include/xen/device_tree.h
+++ b/xen/include/xen/device_tree.h
@@ -94,8 +94,10 @@ struct dt_device_node {
     /* IOMMU specific fields */
     bool is_protected;
 
+#ifdef CONFIG_STATIC_EVTCHN
     /* HACK: Remove this if there is a need of space */
     bool static_evtchn_created;
+#endif
 
     /*
      * The main purpose of this list is to link the structure in the list
@@ -366,18 +368,6 @@ static inline bool dt_property_name_is_equal(const struct dt_property *pp,
     return !dt_prop_cmp(pp->name, name);
 }
 
-static inline void
-dt_device_set_static_evtchn_created(struct dt_device_node *device)
-{
-    device->static_evtchn_created = true;
-}
-
-static inline bool
-dt_device_static_evtchn_created(const struct dt_device_node *device)
-{
-    return device->static_evtchn_created;
-}
-
 /**
  * dt_find_compatible_node - Find a node based on type and one of the
  *                           tokens in its "compatible" property
-- 
2.25.1

Hi Michal

On 29/11/2023 16:34, Michal Orzel wrote:
> Move static event channel feature related code to a separate module
> (static-evtchn.{c,h}) in the spirit of fine granular configuration, so
> that the feature can be disabled if not needed.
> 
> Introduce Kconfig option CONFIG_STATIC_EVTCHN, enabled by default (to
> keep the current behavior) dependent on CONFIG_DOM0LESS. While it could
> be possible to create a loopback connection for dom0 only, this use case
> does not really need this feature and all the docs and commit messages
> refer explicitly to the use in dom0less system.
> 
> The only function visible externally is alloc_static_evtchn(), so move
> the prototype to static-evtchn.h and provide a stub in case a feature
> is disabled. Guard static_evtchn_created in struct dt_device_node and
> move its helpers to static-evtchn.h.

I guess this is a matter of taste, but I think 
dt_device_set_static_evtchn_created() and 
dt_device_static_evtchn_created() are better suited in device_tree.h 
because they are touching a field in the device tree structure.

This would stay consistent with the helper dt_device_set_protected() 
which is only used by the IOMMU code yet is defined in device_tree.h.

That said, I could settle on defining the two helpers in the *.c 
directly because they are not meant to be used outside of a single *.c.

Simarly...

> diff --git a/xen/arch/arm/include/asm/static-evtchn.h b/xen/arch/arm/include/asm/static-evtchn.h
> new file mode 100644
> index 000000000000..472673fae345
> --- /dev/null
> +++ b/xen/arch/arm/include/asm/static-evtchn.h
> @@ -0,0 +1,41 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +
> +#ifndef __ASM_STATIC_EVTCHN_H_
> +#define __ASM_STATIC_EVTCHN_H_
> +
> +#ifdef CONFIG_STATIC_EVTCHN
> +
> +#include <xen/device_tree.h>
> +
> +#define STATIC_EVTCHN_NODE_SIZE_CELLS 2

... this used to be defined in domain_build.c. AFAICT the only use is 
now in static-evtchn.c. So why is it defined in the header?

If this is moved in the *.c, then the header static-evtchn.h would only 
contain alloc_static_evtchn(). This could be moved in domain_build.h or 
setup.h (I don't have any preference).

Cheers,

-- 
Julien Grall

Hi Julien,

On 29/11/2023 18:17, Julien Grall wrote:
> 
> 
> Hi Michal
> 
> On 29/11/2023 16:34, Michal Orzel wrote:
>> Move static event channel feature related code to a separate module
>> (static-evtchn.{c,h}) in the spirit of fine granular configuration, so
>> that the feature can be disabled if not needed.
>>
>> Introduce Kconfig option CONFIG_STATIC_EVTCHN, enabled by default (to
>> keep the current behavior) dependent on CONFIG_DOM0LESS. While it could
>> be possible to create a loopback connection for dom0 only, this use case
>> does not really need this feature and all the docs and commit messages
>> refer explicitly to the use in dom0less system.
>>
>> The only function visible externally is alloc_static_evtchn(), so move
>> the prototype to static-evtchn.h and provide a stub in case a feature
>> is disabled. Guard static_evtchn_created in struct dt_device_node and
>> move its helpers to static-evtchn.h.
> 
> I guess this is a matter of taste, but I think
> dt_device_set_static_evtchn_created() and
> dt_device_static_evtchn_created() are better suited in device_tree.h
> because they are touching a field in the device tree structure.
> 
> This would stay consistent with the helper dt_device_set_protected()
> which is only used by the IOMMU code yet is defined in device_tree.h.
Good point about consistency. I will keep the helpers in device_tree.h + add guards.

> 
> That said, I could settle on defining the two helpers in the *.c
> directly because they are not meant to be used outside of a single *.c.
> 
> Simarly...
> 
>> diff --git a/xen/arch/arm/include/asm/static-evtchn.h b/xen/arch/arm/include/asm/static-evtchn.h
>> new file mode 100644
>> index 000000000000..472673fae345
>> --- /dev/null
>> +++ b/xen/arch/arm/include/asm/static-evtchn.h
>> @@ -0,0 +1,41 @@
>> +/* SPDX-License-Identifier: GPL-2.0-only */
>> +
>> +#ifndef __ASM_STATIC_EVTCHN_H_
>> +#define __ASM_STATIC_EVTCHN_H_
>> +
>> +#ifdef CONFIG_STATIC_EVTCHN
>> +
>> +#include <xen/device_tree.h>
>> +
>> +#define STATIC_EVTCHN_NODE_SIZE_CELLS 2
> 
> ... this used to be defined in domain_build.c. AFAICT the only use is
> now in static-evtchn.c. So why is it defined in the header?
> 
> If this is moved in the *.c, then the header static-evtchn.h would only
> contain alloc_static_evtchn(). This could be moved in domain_build.h or
> setup.h (I don't have any preference).
Apart from a prototype, we still need a stub. Therefore I would prefer to still have a header (will
be needed for future upgrades e.g. port exposure in fdt) and move the prototype and a stub there (the macro
I can move to *.c). It just looks better for me + we reduce ifdefery in domain_build/setup.h.
Would you be ok with that?

~Michal

Hi Michal,

On 29/11/2023 18:41, Michal Orzel wrote:
> On 29/11/2023 18:17, Julien Grall wrote:
>> That said, I could settle on defining the two helpers in the *.c
>> directly because they are not meant to be used outside of a single *.c.
>>
>> Simarly...
>>
>>> diff --git a/xen/arch/arm/include/asm/static-evtchn.h b/xen/arch/arm/include/asm/static-evtchn.h
>>> new file mode 100644
>>> index 000000000000..472673fae345
>>> --- /dev/null
>>> +++ b/xen/arch/arm/include/asm/static-evtchn.h
>>> @@ -0,0 +1,41 @@
>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>> +
>>> +#ifndef __ASM_STATIC_EVTCHN_H_
>>> +#define __ASM_STATIC_EVTCHN_H_
>>> +
>>> +#ifdef CONFIG_STATIC_EVTCHN
>>> +
>>> +#include <xen/device_tree.h>
>>> +
>>> +#define STATIC_EVTCHN_NODE_SIZE_CELLS 2
>>
>> ... this used to be defined in domain_build.c. AFAICT the only use is
>> now in static-evtchn.c. So why is it defined in the header?
>>
>> If this is moved in the *.c, then the header static-evtchn.h would only
>> contain alloc_static_evtchn(). This could be moved in domain_build.h or
>> setup.h (I don't have any preference).
> Apart from a prototype, we still need a stub. Therefore I would prefer to still have a header (will
> be needed for future upgrades e.g. port exposure in fdt) and move the prototype and a stub there (the macro
> I can move to *.c). It just looks better for me + we reduce ifdefery in domain_build/setup.h.
> Would you be ok with that?

I don't much like headers containing just one prototype. But I can live 
with them if you think more will be added.

Cheers,

-- 
Julien Grall

On 29/11/2023 18:54, Julien Grall wrote:
> 
> 
> Hi Michal,
> 
> On 29/11/2023 18:41, Michal Orzel wrote:
>> On 29/11/2023 18:17, Julien Grall wrote:
>>> That said, I could settle on defining the two helpers in the *.c
>>> directly because they are not meant to be used outside of a single *.c.
>>>
>>> Simarly...
>>>
>>>> diff --git a/xen/arch/arm/include/asm/static-evtchn.h b/xen/arch/arm/include/asm/static-evtchn.h
>>>> new file mode 100644
>>>> index 000000000000..472673fae345
>>>> --- /dev/null
>>>> +++ b/xen/arch/arm/include/asm/static-evtchn.h
>>>> @@ -0,0 +1,41 @@
>>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>>> +
>>>> +#ifndef __ASM_STATIC_EVTCHN_H_
>>>> +#define __ASM_STATIC_EVTCHN_H_
>>>> +
>>>> +#ifdef CONFIG_STATIC_EVTCHN
>>>> +
>>>> +#include <xen/device_tree.h>
>>>> +
>>>> +#define STATIC_EVTCHN_NODE_SIZE_CELLS 2
>>>
>>> ... this used to be defined in domain_build.c. AFAICT the only use is
>>> now in static-evtchn.c. So why is it defined in the header?
>>>
>>> If this is moved in the *.c, then the header static-evtchn.h would only
>>> contain alloc_static_evtchn(). This could be moved in domain_build.h or
>>> setup.h (I don't have any preference).
>> Apart from a prototype, we still need a stub. Therefore I would prefer to still have a header (will
>> be needed for future upgrades e.g. port exposure in fdt) and move the prototype and a stub there (the macro
>> I can move to *.c). It just looks better for me + we reduce ifdefery in domain_build/setup.h.
>> Would you be ok with that?
> 
> I don't much like headers containing just one prototype. But I can live
> with them if you think more will be added.
I can at least think of adding support for discovering static ports.
Thank you for meeting me halfway.

~Michal