drivers: vhost: Add Xen-MMIO VIRTIO backend #91605

soburi · 2025-06-13T20:46:35Z

This PR introduces the VHost driver framework for Zephyr, enabling VIRTIO backend implementations with initial support for Xen hypervisor environments.

This means we can use(create) Zephyr as VIRTIO devices in a Xen (and potentially other hypervisors).

VHost Driver Framework

Standard VHost API: Unified interface for VirtIO backend implementations
vringh Integration: Host-side VirtIO ring handling based on Linux kernel design

Xen MMIO VIRTIO Backend

Implementing VIRTIO-MMIO in a Xen environment using the VHost driver framework

Enhanced Xen Infrastructure

Some enhancements of Xen functions

XenStore Integration: Key-value store access for domain configuration

Add more hypercall wrapper: DM-op, scheduler, ...

note:
This pr needs #91643 finished.

github-actions · 2025-06-13T20:47:50Z

The following west manifest projects have changed revision in this Pull Request:

Name	Old Revision	New Revision	Diff
xen 🆕	N/A (Added)	soburi/xen@`24c28d2` (`add_more_hypercall`)	N/A

⛔ DNM label due to: 1 added project

Note: This message is automatically posted and updated by the Manifest GitHub Action.

kartben · 2025-06-13T20:54:04Z

drivers/virtio/Kconfig

@@ -22,6 +22,21 @@ config VIRTIO_MMIO
 	help
 	  Enable options for VIRTIO over MMIO

+config VIRTIO_XEN_MMIO_BACKEND


you have both VIRTIO_XEN_MMIO_BACKEND and XEN_VIRTIO_MMIO_BACKEND :)

s/VIRTIO_XEN_MMIO_BACKEND/XEN_VIRTIO_MMIO_BACKEND/

Thank you for your review.
We will address it gradually.
As you can see, the code is horribly messy at the moment,
and as I said in the title, a lot of updates are still needed.
You might want to wait a little longer to take a proper look.

If anything, this status is an indication that I'm making progress on the tasks, and this is like a report that we have passed the proof-of-concept level.

I think there are some people who are interested in safety features who is interested this.

Add the Xen include files as a module. Signed-off-by: TOKITA Hiroshi <[email protected]>

Currently, the MIT-licensed Xen public headers were incorporated under the `include/zephyr/xen/public`. However, the recent policy is to not include sources other than Apache-2 in the main tree, so we will externalize them. Signed-off-by: TOKITA Hiroshi <[email protected]>

Update Xen module. Signed-off-by: TOKITA Hiroshi <[email protected]>

Add HYPERVISOR_dm_op hypercall implementation for ARM64 architecture. This hypercall is required for device model operations in Xen virtualization environment. Signed-off-by: TOKITA Hiroshi <[email protected]>

Add more hypercall wrappers for supporting virtio-mmio backend. - dmop - dmop_create_ioreq_server HYPERVISOR_dm_op(XEN_DMOP_create_ioreq_server) - dmop_map_io_range_to_ioreq_server HYPERVISOR_dm_op(XEN_DMOP_map_io_range_to_ioreq_server) - dmop_set_ioreq_server_state: HYPERVISOR_dm_op(XEN_DMOP_set_ioreq_server_state) - dmop_nr_vcpus: HYPERVISOR_dm_op(XEN_DMOP_nr_vcpus) - dmop_set_irq_level: HYPERVISOR_dm_op(XEN_DMOP_set_irq_level) - memory - xendom_acquire_resource: HYPERVISOR_memory_op(XENMEM_acquire_resource) - sched - sched_poll: HYPERVISOR_sched_op(SCHEDOP_poll) Signed-off-by: TOKITA Hiroshi <[email protected]>

Add grant table functionality for Xen virtualization support. Implements grant table setup, mapping, and reference management for memory sharing between domains. Signed-off-by: TOKITA Hiroshi <[email protected]>

Move macros derived from the virtio specification to a shared include. Signed-off-by: TOKITA Hiroshi <[email protected]>

Add functions for accessing XenStore. Currently not support write feature. Signed-off-by: TOKITA Hiroshi <[email protected]>

This introduces the vhost driver framework, providing standard APIs for VIRTIO backend implementations in Zephyr. Includes vringh utility for host-side VIRTIO ring processing based on Linux kernel implementation. Signed-off-by: TOKITA Hiroshi <[email protected]>

Implements VirtIO backend over Xen MMIO interface Signed-off-by: TOKITA Hiroshi <[email protected]>

Add sample application demonstrating vhost driver usage for VIRTIO backend implementations. Includes basic setup and configuration examples for Xen MMIO VirtIO backend. Signed-off-by: TOKITA Hiroshi <[email protected]>

firscity

Thank you for proposed changes and your activity, I appreciate that and will try to assist you with advises.

Unfortunately, I need to pause my review on following commit - "drivers: xen: add basic XenStore access features" due to a lack of time. I strongly suggest you to divide this PR in few smaller. Some of the changes may be easily merged already if it will have a good explanation.

I see no major issues with reviewing and merging this changes step by step:

dm_op
adding wrapper functions
gnttab change (if you will explain why it is actually needed right inside a driver)
Virtion config
Vhost
Virtio MMIO
sample

Regarding xenstore.c - what you implemented actually called xenbus (xenstore - aka "filesystem" and server for Dom0, "xenbus" - interface between domains through the ring buffer). It is very complex thing and require a lot of review, I believe it should be done in separate PR.

FYI regarding xenstore (for Zephyr Dom0): after some discussions we decided to move it to separate project instead of having it as a driver (and potentially move it to userspace) - please check zephyr-xenlib/xenstore-srv

firscity · 2025-08-08T08:19:45Z

include/zephyr/arch/arm64/hypercall.h

@@ -6,13 +6,18 @@
 #ifndef ZEPHYR_INCLUDE_ARCH_ARM64_HYPERCALL_H_
 #define ZEPHYR_INCLUDE_ARCH_ARM64_HYPERCALL_H_

+#include <xen/public/xen.h>
+
+struct xen_dm_op_buf;


What is the point for this one? I believe there should be just #include <zephyr/xen/dmop.h> on the top of the file

firscity · 2025-08-08T11:59:22Z

arch/arm64/core/xen/hypercall.S

@@ -23,6 +23,7 @@ HYPERCALL(sched_op);
 HYPERCALL(event_channel_op);
 HYPERCALL(hvm_op);
 HYPERCALL(memory_op);
+HYPERCALL(dm_op);


Generic comment regarding commit message for arch: arm64: core: xen: hypercall: Add dm_op hypercall:

You need to not only describe what was added, but also explain why it is needed in Zephyr and how it will be used.

firscity · 2025-08-08T12:14:19Z

include/zephyr/xen/dmop.h

+int dmop_map_io_range_to_ioreq_server(domid_t domid, ioservid_t id, uint32_t type, uint64_t start,
+				      uint64_t end);
+
+int dmop_unmap_io_range_from_ioreq_server(domid_t domid, ioservid_t id, uint32_t type,


This one has missing documentation comment. I understand that this and previous are opposite to each other, but when docs will be generated it will look strange.

firscity · 2025-08-08T12:15:10Z

include/zephyr/xen/dmop.h

+ */
+int dmop_create_ioreq_server(domid_t domid, uint8_t handle_bufioreq, ioservid_t *id);
+
+int dmop_destroy_ioreq_server(domid_t domid, ioservid_t id);


Same for this one(as for dmop_unmap_io_range_from_ioreq_server)

firscity · 2025-08-08T12:17:28Z

drivers/xen/CMakeLists.txt

@@ -5,5 +5,7 @@ zephyr_sources(hvm.c)
 zephyr_sources(events.c)
 zephyr_sources_ifdef(CONFIG_XEN_GRANT_TABLE gnttab.c)
 zephyr_sources(memory.c)
+zephyr_sources(sched.c)
+zephyr_sources(dmop.c)


Overall comment for commit drivers: xen: add more hypercall wrapper:
I strongly suggest you to split this commit into 3 separate commits and add more description regarding how it will be used in further development. Also, I believe that sched.c and dmop.c should be guarded with separate CONFIG to have a possibility to disable it.

firscity · 2025-08-08T12:51:06Z

drivers/xen/gnttab.c

-void gnttab_put_page(void *page_addr)
+void *gnttab_get_page(void)
+{
+	return gnttab_get_pages(1);


It looks like half-way change, I believe if plural form is actually needed, that it is better to fix it everywhere and not leave things like this.

firscity · 2025-08-08T12:53:49Z

drivers/xen/gnttab.c

+		reservation.nr_extents = nr_extents;
+		set_xen_guest_handle(reservation.extent_start, &page);
+
+		ret = HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation);


This hypercall supports multiple pages out of the box, you don't need a loop here. Please, pay attention to nr_extents field, it can be done it 1 action.

firscity · 2025-08-08T12:55:00Z

drivers/xen/gnttab.c

@@ -286,8 +305,7 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops, unsigned int cou
 	return HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, unmap_ops, count);
 }

-
-static const char * const gnttab_error_msgs[] = GNTTABOP_error_msgs;
+static const char *const gnttab_error_msgs[] = GNTTABOP_error_msgs;


Style fixes should be in separate commit.

firscity · 2025-08-08T12:56:06Z

include/zephyr/xen/gnttab.h

@@ -50,13 +50,17 @@ int32_t gnttab_alloc_and_grant(void **map, bool readonly);
 */
 void *gnttab_get_page(void);

+void *gnttab_get_pages(unsigned int npages);


As I said previously, I believe that user should see only 1 form for this (and it should have a documentation block above)

firscity · 2025-08-08T13:07:29Z

drivers/virtio/virtio_common.c

@@ -7,6 +7,7 @@
 #include <zephyr/logging/log.h>
 #include <zephyr/sys/byteorder.h>
 #include <zephyr/drivers/virtio.h>
+#include <zephyr/drivers/virtio/virtio_config.h>


Comment for drivers: virtio: Separate macros into a common header:
I am not responsible for this part of code, but I have no idea why did you change it, commit message definitely should describe it better.

github-actions bot added the manifest label Jun 13, 2025

kartben reviewed Jun 13, 2025

View reviewed changes

soburi added area: Xen Platform area: VIRTIO labels Jun 13, 2025

soburi force-pushed the xen-virtio-backend branch 6 times, most recently from 9451269 to 144315f Compare June 15, 2025 23:30

soburi changed the title ~~drivers: virtio: Add Xen virtio-mmio backend~~ drivers: virtio: Add Xen VIRTIO-MMIO backend Jun 17, 2025

soburi force-pushed the xen-virtio-backend branch from 144315f to 91ba84f Compare June 18, 2025 23:13

soburi force-pushed the xen-virtio-backend branch 3 times, most recently from 4741c1a to 141d60f Compare July 17, 2025 10:53

soburi force-pushed the xen-virtio-backend branch 9 times, most recently from 21b1787 to 98d1f30 Compare July 27, 2025 07:40

soburi added the DNM This PR should not be merged (Do Not Merge) label Jul 27, 2025

soburi force-pushed the xen-virtio-backend branch 2 times, most recently from d64559b to 986ef95 Compare July 29, 2025 13:35

zephyrbot requested review from carlocaione, firscity, fkokosinski, ithinuel, kgugala, lorc, luca-fancellu, nashif, npitre, povergoing, SgrrZhf, tgorochowik and wearyzen August 2, 2025 00:44

zephyrbot assigned kartben Aug 2, 2025

soburi force-pushed the xen-virtio-backend branch 2 times, most recently from bf1eefc to 5fed92a Compare August 5, 2025 10:00

soburi added 11 commits August 5, 2025 21:28

[DNM] manifest: add Xen module

9a620d7

Add the Xen include files as a module. Signed-off-by: TOKITA Hiroshi <[email protected]>

[DNM] manifest: Update Xen module

2bb5f42

Update Xen module. Signed-off-by: TOKITA Hiroshi <[email protected]>

arch: arm64: core: xen: hypercall: Add dm_op hypercall

710d11d

Add HYPERVISOR_dm_op hypercall implementation for ARM64 architecture. This hypercall is required for device model operations in Xen virtualization environment. Signed-off-by: TOKITA Hiroshi <[email protected]>

drivers: xen: implement grant table operations

0ef7694

Add grant table functionality for Xen virtualization support. Implements grant table setup, mapping, and reference management for memory sharing between domains. Signed-off-by: TOKITA Hiroshi <[email protected]>

drivers: virtio: Separate macros into a common header

834a8a3

Move macros derived from the virtio specification to a shared include. Signed-off-by: TOKITA Hiroshi <[email protected]>

drivers: xen: add basic XenStore access features

071ee76

Add functions for accessing XenStore. Currently not support write feature. Signed-off-by: TOKITA Hiroshi <[email protected]>

drivers: vhost: add Xen MMIO VirtIO backend

af226c7

Implements VirtIO backend over Xen MMIO interface Signed-off-by: TOKITA Hiroshi <[email protected]>

samples: drivers: virtualization: add vhost sample application

9376df3

Add sample application demonstrating vhost driver usage for VIRTIO backend implementations. Includes basic setup and configuration examples for Xen MMIO VirtIO backend. Signed-off-by: TOKITA Hiroshi <[email protected]>

soburi force-pushed the xen-virtio-backend branch from 5fed92a to 9376df3 Compare August 5, 2025 12:38

soburi mentioned this pull request Aug 5, 2025

modules: add and migrate to the Xen module #91643

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drivers: vhost: Add Xen-MMIO VIRTIO backend #91605

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VHost Driver Framework

Xen MMIO VIRTIO Backend

Enhanced Xen Infrastructure

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