Re: [PATCH v2] vhost: introduce mdev based hardware backend

From: Jason Wang
Date: Wed Oct 23 2019 - 01:46:53 EST


On 2019/10/23 äå11:02, Tiwei Bie wrote:
On Tue, Oct 22, 2019 at 09:30:16PM +0800, Jason Wang wrote:
On 2019/10/22 äå5:52, Tiwei Bie wrote:
This patch introduces a mdev based hardware vhost backend.
This backend is built on top of the same abstraction used
in virtio-mdev and provides a generic vhost interface for
userspace to accelerate the virtio devices in guest.

This backend is implemented as a mdev device driver on top
of the same mdev device ops used in virtio-mdev but using
a different mdev class id, and it will register the device
as a VFIO device for userspace to use. Userspace can setup
the IOMMU with the existing VFIO container/group APIs and
then get the device fd with the device name. After getting
the device fd of this device, userspace can use vhost ioctls
to setup the backend.

Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
---
This patch depends on below series:
https://lkml.org/lkml/2019/10/17/286

v1 -> v2:
- Replace _SET_STATE with _SET_STATUS (MST);
- Check status bits at each step (MST);
- Report the max ring size and max number of queues (MST);
- Add missing MODULE_DEVICE_TABLE (Jason);
- Only support the network backend w/o multiqueue for now;

Any idea on how to extend it to support devices other than net? I think we
want a generic API or an API that could be made generic in the future.

Do we want to e.g having a generic vhost mdev for all kinds of devices or
introducing e.g vhost-net-mdev and vhost-scsi-mdev?
One possible way is to do what vhost-user does. I.e. Apart from
the generic ring, features, ... related ioctls, we also introduce
device specific ioctls when we need them. As vhost-mdev just needs
to forward configs between parent and userspace and even won't
cache any info when possible,


So it looks to me this is only possible if we expose e.g set_config and get_config to userspace.


I think it might be better to do
this in one generic vhost-mdev module.


Looking at definitions of VhostUserRequest in qemu, it mixed generic API with device specific API. If we want go this ways (a generic vhost-mdev), more questions needs to be answered:

1) How could userspace know which type of vhost it would use? Do we need to expose virtio subsystem device in for userspace this case?

2) That generic vhost-mdev module still need to filter out unsupported ioctls for a specific type. E.g if it probes a net device, it should refuse API for other type. This in fact a vhost-mdev-net but just not modularize it on top of vhost-mdev.




- Some minor fixes and improvements;
- Rebase on top of virtio-mdev series v4;

RFC v4 -> v1:
- Implement vhost-mdev as a mdev device driver directly and
connect it to VFIO container/group. (Jason);
- Pass ring addresses as GPAs/IOVAs in vhost-mdev to avoid
meaningless HVA->GPA translations (Jason);

RFC v3 -> RFC v4:
- Build vhost-mdev on top of the same abstraction used by
virtio-mdev (Jason);
- Introduce vhost fd and pass VFIO fd via SET_BACKEND ioctl (MST);

RFC v2 -> RFC v3:
- Reuse vhost's ioctls instead of inventing a VFIO regions/irqs
based vhost protocol on top of vfio-mdev (Jason);

RFC v1 -> RFC v2:
- Introduce a new VFIO device type to build a vhost protocol
on top of vfio-mdev;

drivers/vfio/mdev/mdev_core.c | 12 +
drivers/vhost/Kconfig | 9 +
drivers/vhost/Makefile | 3 +
drivers/vhost/mdev.c | 415 ++++++++++++++++++++++++++++++++++
include/linux/mdev.h | 3 +
include/uapi/linux/vhost.h | 13 ++
6 files changed, 455 insertions(+)
create mode 100644 drivers/vhost/mdev.c

diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
index 5834f6b7c7a5..2963f65e6648 100644
--- a/drivers/vfio/mdev/mdev_core.c
+++ b/drivers/vfio/mdev/mdev_core.c
@@ -69,6 +69,18 @@ void mdev_set_virtio_ops(struct mdev_device *mdev,
}
EXPORT_SYMBOL(mdev_set_virtio_ops);
+/* Specify the vhost device ops for the mdev device, this
+ * must be called during create() callback for vhost mdev device.
+ */
+void mdev_set_vhost_ops(struct mdev_device *mdev,
+ const struct virtio_mdev_device_ops *vhost_ops)
+{
+ WARN_ON(mdev->class_id);
+ mdev->class_id = MDEV_CLASS_ID_VHOST;
+ mdev->device_ops = vhost_ops;
+}
+EXPORT_SYMBOL(mdev_set_vhost_ops);
+
const void *mdev_get_dev_ops(struct mdev_device *mdev)
{
return mdev->device_ops;
diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
index 3d03ccbd1adc..7b5c2f655af7 100644
--- a/drivers/vhost/Kconfig
+++ b/drivers/vhost/Kconfig
@@ -34,6 +34,15 @@ config VHOST_VSOCK
To compile this driver as a module, choose M here: the module will be called
vhost_vsock.
+config VHOST_MDEV
+ tristate "Vhost driver for Mediated devices"
+ depends on EVENTFD && VFIO && VFIO_MDEV
+ select VHOST
+ default n
+ ---help---
+ Say M here to enable the vhost_mdev module for use with
+ the mediated device based hardware vhost accelerators.
+
config VHOST
tristate
---help---
diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
index 6c6df24f770c..ad9c0f8c6d8c 100644
--- a/drivers/vhost/Makefile
+++ b/drivers/vhost/Makefile
@@ -10,4 +10,7 @@ vhost_vsock-y := vsock.o
obj-$(CONFIG_VHOST_RING) += vringh.o
+obj-$(CONFIG_VHOST_MDEV) += vhost_mdev.o
+vhost_mdev-y := mdev.o
+
obj-$(CONFIG_VHOST) += vhost.o
diff --git a/drivers/vhost/mdev.c b/drivers/vhost/mdev.c
new file mode 100644
index 000000000000..5f9cae61018c
--- /dev/null
+++ b/drivers/vhost/mdev.c
@@ -0,0 +1,415 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2018-2019 Intel Corporation.
+ */
+
+#include <linux/compat.h>
+#include <linux/kernel.h>
+#include <linux/miscdevice.h>
+#include <linux/mdev.h>
+#include <linux/module.h>
+#include <linux/vfio.h>
+#include <linux/vhost.h>
+#include <linux/virtio_mdev.h>
+#include <linux/virtio_ids.h>
+
+#include "vhost.h"
+
+/* Currently, only network backend w/o multiqueue is supported. */
+#define VHOST_MDEV_VQ_MAX 2
+
+struct vhost_mdev {
+ /* The lock is to protect this structure. */
+ struct mutex mutex;
+ struct vhost_dev dev;
+ struct vhost_virtqueue *vqs;
+ int nvqs;
+ u64 status;
+ u64 features;
+ u64 acked_features;
+ bool opened;
+ struct mdev_device *mdev;
+};
+
+static void handle_vq_kick(struct vhost_work *work)
+{
+ struct vhost_virtqueue *vq = container_of(work, struct vhost_virtqueue,
+ poll.work);
+ struct vhost_mdev *m = container_of(vq->dev, struct vhost_mdev, dev);
+ const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(m->mdev);
+
+ ops->kick_vq(m->mdev, vq - m->vqs);
+}
+
+static irqreturn_t vhost_mdev_virtqueue_cb(void *private)
+{
+ struct vhost_virtqueue *vq = private;
+ struct eventfd_ctx *call_ctx = vq->call_ctx;
+
+ if (call_ctx)
+ eventfd_signal(call_ctx, 1);
+ return IRQ_HANDLED;
+}
+
+static void vhost_mdev_reset(struct vhost_mdev *m)
+{
+ struct mdev_device *mdev = m->mdev;
+ const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(mdev);
+
+ m->status = 0;
+ return ops->set_status(mdev, m->status);
+}
+
+static long vhost_mdev_get_status(struct vhost_mdev *m, u8 __user *statusp)
+{
+ const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(m->mdev);
+ struct mdev_device *mdev = m->mdev;
+ u8 status;
+
+ status = ops->get_status(mdev);
+ m->status = status;
+
+ if (copy_to_user(statusp, &status, sizeof(status)))
+ return -EFAULT;
+
+ return 0;
+}
+
+static long vhost_mdev_set_status(struct vhost_mdev *m, u8 __user *statusp)
+{
+ const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(m->mdev);
+ struct mdev_device *mdev = m->mdev;
+ u8 status;
+
+ if (copy_from_user(&status, statusp, sizeof(status)))
+ return -EFAULT;
+
+ /*
+ * Userspace shouldn't remove status bits unless reset the
+ * status to 0.
+ */
+ if (status != 0 && (m->status & ~status) != 0)
+ return -EINVAL;

We don't cache vq ready information but we cache status and features here,
any reason for this?
+1, I think it's better to not cache any unnecessary
information in vhost-mdev.


+
+ ops->set_status(mdev, status);
+ m->status = ops->get_status(mdev);
+
+ return 0;
+}
+
+static long vhost_mdev_get_features(struct vhost_mdev *m, u64 __user *featurep)
+{
+ if (copy_to_user(featurep, &m->features, sizeof(m->features)))
+ return -EFAULT;

As discussed in previous version do we need to filter out MQ feature here?
I think it's more straightforward to let the parent drivers to
filter out the unsupported features. Otherwise it would be tricky
when we want to add more features in vhost-mdev module,


It's as simple as remove the feature from blacklist?


i.e. if
the parent drivers may expose unsupported features and relay on
vhost-mdev to filter them out, these features will be exposed
to userspace automatically when they are enabled in vhost-mdev
in the future.


The issue is, it's only that vhost-mdev knows its own limitation. E.g in this patch, vhost-mdev only implements a subset of transport API, but parent doesn't know about that.

Still MQ as an example, there's no way (or no need) for parent to know that vhost-mdev does not support MQ. And this allows old kenrel to work with new parent drivers.

So basically we have three choices here:

1) Implement what vhost-user did and implement a generic vhost-mdev (but may still have lots of device specific code). To support advanced feature which requires the access to config, still lots of API that needs to be added.

2) Implement what vhost-kernel did, have a generic vhost-mdev driver and a vhost bus on top for match a device specific API e.g vhost-mdev-net. We still have device specific API but limit them only to device specific module. Still require new ioctls for advanced feature like MQ.

3) Simply expose all virtio-mdev transport to userspace. A generic module without any type specific code (like virtio-mdev). No need dedicated API for e.g MQ. But then the API will look much different than current vhost did.

Consider the limitation of 1) I tend to choose 2 or 3. What's you opinion?




+ return 0;
+}
+
+static long vhost_mdev_set_features(struct vhost_mdev *m, u64 __user *featurep)
+{
+ const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(m->mdev);
+ struct mdev_device *mdev = m->mdev;
+ u64 features;
+
+ /*
+ * It's not allowed to change the features after they have
+ * been negotiated.
+ */
+ if (m->status & VIRTIO_CONFIG_S_FEATURES_OK)
+ return -EPERM;

-EBUSY?
Yeah, definitely.


+
+ if (copy_from_user(&features, featurep, sizeof(features)))
+ return -EFAULT;
+
+ if (features & ~m->features)
+ return -EINVAL;
+
+ m->acked_features = features;
+ if (ops->set_features(mdev, m->acked_features))
+ return -ENODEV;

-EINVAL should be better, this would be more obvious for parent that wants
to force any feature.
+1. Agree.


+
+ return 0;
+}
+
+static long vhost_mdev_get_vring_num(struct vhost_mdev *m, u16 __user *argp)
+{
+ const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(m->mdev);
+ struct mdev_device *mdev = m->mdev;
+ u16 num;
+
+ num = ops->get_vq_num_max(mdev);
+
+ if (copy_to_user(argp, &num, sizeof(num)))
+ return -EFAULT;
+ return 0;
+}
+
+static long vhost_mdev_get_queue_num(struct vhost_mdev *m, u32 __user *argp)
+{
+ u32 nvqs = m->nvqs;
+
+ if (copy_to_user(argp, &nvqs, sizeof(nvqs)))
+ return -EFAULT;
+ return 0;
+}
+
+static long vhost_mdev_vring_ioctl(struct vhost_mdev *m, unsigned int cmd,
+ void __user *argp)
+{
+ const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(m->mdev);
+ struct mdev_device *mdev = m->mdev;
+ struct virtio_mdev_callback cb;
+ struct vhost_virtqueue *vq;
+ struct vhost_vring_state s;
+ u32 idx;
+ long r;
+
+ r = get_user(idx, (u32 __user *)argp);
+ if (r < 0)
+ return r;
+ if (idx >= m->nvqs)
+ return -ENOBUFS;
+
+ /*
+ * It's not allowed to detect and program vqs before
+ * features negotiation or after enabling driver.
+ */
+ if (!(m->status & VIRTIO_CONFIG_S_FEATURES_OK) ||
+ (m->status & VIRTIO_CONFIG_S_DRIVER_OK))
+ return -EPERM;

So the question is: is it better to do this in parent or not?
I think it will duplicate the generic code in each parent.


+
+ vq = &m->vqs[idx];
+
+ if (cmd == VHOST_MDEV_SET_VRING_ENABLE) {
+ if (copy_from_user(&s, argp, sizeof(s)))
+ return -EFAULT;
+ ops->set_vq_ready(mdev, idx, s.num);
+ return 0;
+ }
+
+ /*
+ * It's not allowed to detect and program vqs with
+ * vqs enabled.
+ */
+ if (ops->get_vq_ready(mdev, idx))
+ return -EPERM;
+
+ if (cmd == VHOST_GET_VRING_BASE)
+ vq->last_avail_idx = ops->get_vq_state(m->mdev, idx);
+
+ r = vhost_vring_ioctl(&m->dev, cmd, argp);
+ if (r)
+ return r;
+
+ switch (cmd) {
+ case VHOST_SET_VRING_ADDR:
+ /*
+ * In vhost-mdev, the ring addresses set by userspace should
+ * be the DMA addresses within the VFIO container/group.
+ */
+ if (ops->set_vq_address(mdev, idx, (u64)vq->desc,
+ (u64)vq->avail, (u64)vq->used))
+ r = -ENODEV;
+ break;
+
+ case VHOST_SET_VRING_BASE:
+ if (ops->set_vq_state(mdev, idx, vq->last_avail_idx))
+ r = -ENODEV;
+ break;
+
+ case VHOST_SET_VRING_CALL:
+ if (vq->call_ctx) {
+ cb.callback = vhost_mdev_virtqueue_cb;
+ cb.private = vq;
+ } else {
+ cb.callback = NULL;
+ cb.private = NULL;
+ }
+ ops->set_vq_cb(mdev, idx, &cb);
+ break;
+
+ case VHOST_SET_VRING_NUM:
+ ops->set_vq_num(mdev, idx, vq->num);
+ break;
+ }
+
+ return r;
+}
+
+static int vhost_mdev_open(void *device_data)
+{
+ struct vhost_mdev *m = device_data;
+ struct vhost_dev *dev;
+ struct vhost_virtqueue **vqs;
+ int nvqs, i, r;
+
+ if (!try_module_get(THIS_MODULE))
+ return -ENODEV;
+
+ mutex_lock(&m->mutex);
+
+ if (m->opened) {
+ r = -EBUSY;
+ goto err;
+ }
+
+ nvqs = m->nvqs;
+ vhost_mdev_reset(m);
+
+ memset(&m->dev, 0, sizeof(m->dev));
+ memset(m->vqs, 0, nvqs * sizeof(struct vhost_virtqueue));
+
+ vqs = kmalloc_array(nvqs, sizeof(*vqs), GFP_KERNEL);
+ if (!vqs) {
+ r = -ENOMEM;
+ goto err;
+ }
+
+ dev = &m->dev;
+ for (i = 0; i < nvqs; i++) {
+ vqs[i] = &m->vqs[i];
+ vqs[i]->handle_kick = handle_vq_kick;
+ }
+ vhost_dev_init(dev, vqs, nvqs, 0, 0, 0);
+ m->opened = true;
+ mutex_unlock(&m->mutex);
+
+ return 0;
+
+err:
+ mutex_unlock(&m->mutex);
+ module_put(THIS_MODULE);
+ return r;
+}
+
+static void vhost_mdev_release(void *device_data)
+{
+ struct vhost_mdev *m = device_data;
+
+ mutex_lock(&m->mutex);
+ vhost_mdev_reset(m);
+ vhost_dev_stop(&m->dev);
+ vhost_dev_cleanup(&m->dev);
+
+ kfree(m->dev.vqs);
+ m->opened = false;
+ mutex_unlock(&m->mutex);
+ module_put(THIS_MODULE);
+}
+
+static long vhost_mdev_unlocked_ioctl(void *device_data,
+ unsigned int cmd, unsigned long arg)
+{
+ struct vhost_mdev *m = device_data;
+ void __user *argp = (void __user *)arg;
+ long r;
+
+ mutex_lock(&m->mutex);
+
+ switch (cmd) {
+ case VHOST_MDEV_GET_STATUS:
+ r = vhost_mdev_get_status(m, argp);
+ break;
+ case VHOST_MDEV_SET_STATUS:
+ r = vhost_mdev_set_status(m, argp);
+ break;
+ case VHOST_GET_FEATURES:
+ r = vhost_mdev_get_features(m, argp);
+ break;
+ case VHOST_SET_FEATURES:
+ r = vhost_mdev_set_features(m, argp);
+ break;
+ case VHOST_MDEV_GET_VRING_NUM:
+ r = vhost_mdev_get_vring_num(m, argp);
+ break;
+ case VHOST_MDEV_GET_QUEUE_NUM:
+ r = vhost_mdev_get_queue_num(m, argp);
+ break;

It's not clear to me that how this API will be used by userspace? I think
e.g features without MQ implies the queue num here.
I was thinking about always letting _GET_QUEUE_NUM return
the supported number of queues. For virtio devices other
than virtio-net, can we always expect to have a fixed
default number of queues when there is no MQ feature?


It could be very tricky since each type has its own MQ feature, you probably want a map between device id and its default queue num. But if we don't support MQ, userspace should follow the #queue that is defined by spec. So I still don't see value for this API.

In the future, consider we want to support multiqueue, it's still much tricky than exporting device config space to userspace.




+ default:
+ r = vhost_dev_ioctl(&m->dev, cmd, argp);

I believe having SET_MEM_TABLE/SET_LOG_BASE/SET_LOG_FDÂ is for future
support of those features. If it's true need add some comments on this.
OK.


+ if (r == -ENOIOCTLCMD)
+ r = vhost_mdev_vring_ioctl(m, cmd, argp);
+ }
+
+ mutex_unlock(&m->mutex);
+ return r;
+}
+
+static const struct vfio_device_ops vfio_vhost_mdev_dev_ops = {
+ .name = "vfio-vhost-mdev",
+ .open = vhost_mdev_open,
+ .release = vhost_mdev_release,
+ .ioctl = vhost_mdev_unlocked_ioctl,
+};
+
+static int vhost_mdev_probe(struct device *dev)
+{
+ struct mdev_device *mdev = mdev_from_dev(dev);
+ const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(mdev);
+ struct vhost_mdev *m;
+ int nvqs, r;
+
+ /* Currently, only network backend is supported. */
+ if (ops->get_device_id(mdev) != VIRTIO_ID_NET)
+ return -ENOTSUPP;

If we decide to go with the way of vhost-net-mdev, probably need something
smarter. E.g a vhost bus etc.


+
+ if (ops->get_mdev_features(mdev) != VIRTIO_MDEV_F_VERSION_1)
+ return -ENOTSUPP;
+
+ m = devm_kzalloc(dev, sizeof(*m), GFP_KERNEL | __GFP_RETRY_MAYFAIL);
+ if (!m)
+ return -ENOMEM;
+
+ nvqs = VHOST_MDEV_VQ_MAX;
+ m->nvqs = nvqs;
+
+ m->vqs = devm_kmalloc_array(dev, nvqs, sizeof(struct vhost_virtqueue),
+ GFP_KERNEL);
+ if (!m->vqs)
+ return -ENOMEM;

Is it better to move those allocation to open? Otherwise the memset there
seems strange.
OK.


+
+ r = vfio_add_group_dev(dev, &vfio_vhost_mdev_dev_ops, m);
+ if (r)
+ return r;
+
+ mutex_init(&m->mutex);
+ m->features = ops->get_features(mdev);
+ m->mdev = mdev;
+ return 0;
+}
+
+static void vhost_mdev_remove(struct device *dev)
+{
+ struct vhost_mdev *m;
+
+ m = vfio_del_group_dev(dev);
+ mutex_destroy(&m->mutex);
+}
+
+static const struct mdev_class_id vhost_mdev_match[] = {
+ { MDEV_CLASS_ID_VHOST },
+ { 0 },
+};
+MODULE_DEVICE_TABLE(mdev, vhost_mdev_match);
+
+static struct mdev_driver vhost_mdev_driver = {
+ .name = "vhost_mdev",
+ .probe = vhost_mdev_probe,
+ .remove = vhost_mdev_remove,
+ .id_table = vhost_mdev_match,
+};
+
+static int __init vhost_mdev_init(void)
+{
+ return mdev_register_driver(&vhost_mdev_driver, THIS_MODULE);
+}
+module_init(vhost_mdev_init);
+
+static void __exit vhost_mdev_exit(void)
+{
+ mdev_unregister_driver(&vhost_mdev_driver);
+}
+module_exit(vhost_mdev_exit);
+
+MODULE_VERSION("0.0.1");
+MODULE_LICENSE("GPL v2");
+MODULE_DESCRIPTION("Mediated device based accelerator for virtio");
diff --git a/include/linux/mdev.h b/include/linux/mdev.h
index 13e045e09d3b..6060cdbe6d3e 100644
--- a/include/linux/mdev.h
+++ b/include/linux/mdev.h
@@ -114,6 +114,8 @@ void mdev_set_vfio_ops(struct mdev_device *mdev,
const struct vfio_mdev_device_ops *vfio_ops);
void mdev_set_virtio_ops(struct mdev_device *mdev,
const struct virtio_mdev_device_ops *virtio_ops);
+void mdev_set_vhost_ops(struct mdev_device *mdev,
+ const struct virtio_mdev_device_ops *vhost_ops);
const void *mdev_get_dev_ops(struct mdev_device *mdev);
extern struct bus_type mdev_bus_type;
@@ -131,6 +133,7 @@ struct mdev_device *mdev_from_dev(struct device *dev);
enum {
MDEV_CLASS_ID_VFIO = 1,
MDEV_CLASS_ID_VIRTIO = 2,
+ MDEV_CLASS_ID_VHOST = 3,
/* New entries must be added here */
};
diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index 40d028eed645..dad3c62bd91b 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -116,4 +116,17 @@
#define VHOST_VSOCK_SET_GUEST_CID _IOW(VHOST_VIRTIO, 0x60, __u64)
#define VHOST_VSOCK_SET_RUNNING _IOW(VHOST_VIRTIO, 0x61, int)
+/* VHOST_MDEV specific defines */
+
+/* Get and set the status of the backend. The status bits follow the
+ * same definition of the device status defined in virtio-spec. */
+#define VHOST_MDEV_GET_STATUS _IOW(VHOST_VIRTIO, 0x70, __u8)
+#define VHOST_MDEV_SET_STATUS _IOW(VHOST_VIRTIO, 0x71, __u8)
+/* Enable/disable the ring. */
+#define VHOST_MDEV_SET_VRING_ENABLE _IOW(VHOST_VIRTIO, 0x72, struct vhost_vring_state)
+/* Get the max ring size. */
+#define VHOST_MDEV_GET_VRING_NUM _IOW(VHOST_VIRTIO, 0x73, __u16)
+/* Get the max number of queues. */
+#define VHOST_MDEV_GET_QUEUE_NUM _IOW(VHOST_VIRTIO, 0x74, __u32)

Do we need API for userspace to get backend capability? (that calls
get_mdev_device_features())
Vhost already has the features and backend features ioctls.


Good point, we probably need a new bit for _F_LOG_ALL. But question still, how vhost-mdev know whether the parent support dirty page tracking?

Thanks


In vhost-mdev, it might be better to still just use them to
expose the backend capability to userspace.

Thanks for the review!
Tiwei

Thanks


+
#endif