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

From: Jason Wang <jasowang@redhat.com>
Date: 2019-09-27 03:46:23
Also in: kvm, lkml

On 2019/9/26 下午12:54, 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 <redacted>
---
This patch depends on below series:
https://lkml.org/lkml/2019/9/24/357


Looks pretty nice, comments inline.

quoted hunk ↗ jump to hunk

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/vhost/Kconfig      |   9 +
  drivers/vhost/Makefile     |   3 +
  drivers/vhost/mdev.c       | 381 +++++++++++++++++++++++++++++++++++++
  include/uapi/linux/vhost.h |   8 +
  4 files changed, 401 insertions(+)
  create mode 100644 drivers/vhost/mdev.c

diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
index 3d03ccbd1adc..decf0be8efe9 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..1c12a25b86a2
--- /dev/null
+++ b/drivers/vhost/mdev.c

@@ -0,0 +1,381 @@
+// 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 "vhost.h"
+
+struct vhost_mdev {
+	/* The lock is to protect this structure. */
+	struct mutex mutex;
+	struct vhost_dev dev;
+	struct vhost_virtqueue *vqs;
+	int nvqs;
+	u64 state;
+	u64 features;
+	u64 acked_features;
+	bool opened;
+	struct mdev_device *mdev;
+};
+
+static u8 mdev_get_status(struct mdev_device *mdev)
+{
+	const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(mdev);
+
+	return ops->get_status(mdev);
+}
+
+static void mdev_set_status(struct mdev_device *mdev, u8 status)
+{
+	const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(mdev);
+
+	return ops->set_status(mdev, status);
+}
+
+static void mdev_add_status(struct mdev_device *mdev, u8 status)
+{
+	status |= mdev_get_status(mdev);
+	mdev_set_status(mdev, status);
+}
+
+static void mdev_reset(struct mdev_device *mdev)
+{
+	mdev_set_status(mdev, 0);
+}
+
+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 long vhost_mdev_reset(struct vhost_mdev *m)
+{
+	struct mdev_device *mdev = m->mdev;
+
+	mdev_reset(mdev);
+	mdev_add_status(mdev, VIRTIO_CONFIG_S_ACKNOWLEDGE);
+	mdev_add_status(mdev, VIRTIO_CONFIG_S_DRIVER);
+	return 0;
+}
+
+static long vhost_mdev_start(struct vhost_mdev *m)
+{
+	struct mdev_device *mdev = m->mdev;
+	const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(mdev);
+	struct virtio_mdev_callback cb;
+	struct vhost_virtqueue *vq;
+	int idx;
+
+	ops->set_features(mdev, m->acked_features);
+
+	mdev_add_status(mdev, VIRTIO_CONFIG_S_FEATURES_OK);
+	if (!(mdev_get_status(mdev) & VIRTIO_CONFIG_S_FEATURES_OK))
+		goto reset;
+
+	for (idx = 0; idx < m->nvqs; idx++) {
+		vq = &m->vqs[idx];
+
+		if (!vq->desc || !vq->avail || !vq->used)
+			break;
+
+		if (ops->set_vq_state(mdev, idx, vq->last_avail_idx))
+			goto reset;


If we do set_vq_state() in SET_VRING_BASE, we won't need this step here.

+
+		/*
+		 * In vhost-mdev, userspace should pass ring addresses
+		 * in guest physical addresses when IOMMU is disabled or
+		 * IOVAs when IOMMU is enabled.
+		 */


A question here, consider we're using noiommu mode. If guest physical 
address is passed here, how can a device use that?

I believe you meant "host physical address" here? And it also have the 
implication that the HPA should be continuous (e.g using hugetlbfs).

Thanks

+		if (ops->set_vq_address(mdev, idx, (u64)vq->desc,
+					(u64)vq->avail, (u64)vq->used))
+			goto reset;
+
+		ops->set_vq_num(mdev, idx, vq->num);
+
+		cb.callback = vhost_mdev_virtqueue_cb;
+		cb.private = vq;
+		ops->set_vq_cb(mdev, idx, &cb);
+
+		ops->set_vq_ready(mdev, idx, 1);
+	}
+
+	mdev_add_status(mdev, VIRTIO_CONFIG_S_DRIVER_OK);
+	if (mdev_get_status(mdev) & VIRTIO_CONFIG_S_NEEDS_RESET)
+		goto reset;
+	return 0;
+
+reset:
+	vhost_mdev_reset(m);
+	return -ENODEV;
+}
+
+static long vhost_set_state(struct vhost_mdev *m, u64 __user *statep)
+{
+	u64 state;
+	long r;
+
+	if (copy_from_user(&state, statep, sizeof(state)))
+		return -EFAULT;
+
+	if (state >= VHOST_MDEV_S_MAX)
+		return -EINVAL;
+
+	if (m->state == state)
+		return 0;
+
+	m->state = state;
+
+	switch (m->state) {
+	case VHOST_MDEV_S_RUNNING:
+		r = vhost_mdev_start(m);
+		break;
+	case VHOST_MDEV_S_STOPPED:
+		r = vhost_mdev_reset(m);
+		break;
+	default:
+		r = -EINVAL;
+		break;
+	}
+
+	return r;
+}
+
+static long vhost_get_features(struct vhost_mdev *m, u64 __user *featurep)
+{
+	if (copy_to_user(featurep, &m->features, sizeof(m->features)))
+		return -EFAULT;
+	return 0;
+}
+
+static long vhost_set_features(struct vhost_mdev *m, u64 __user *featurep)
+{
+	u64 features;
+
+	if (copy_from_user(&features, featurep, sizeof(features)))
+		return -EFAULT;
+
+	if (features & ~m->features)
+		return -EINVAL;
+
+	m->acked_features = features;
+
+	return 0;
+}
+
+static long vhost_get_vring_base(struct vhost_mdev *m, void __user *argp)
+{
+	const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(m->mdev);
+	struct vhost_virtqueue *vq;
+	u32 idx;
+	long r;
+
+	r = get_user(idx, (u32 __user *)argp);
+	if (r < 0)
+		return r;
+	if (idx >= m->nvqs)
+		return -ENOBUFS;
+
+	vq = &m->vqs[idx];
+	vq->last_avail_idx = ops->get_vq_state(m->mdev, idx);
+
+	return vhost_vring_ioctl(&m->dev, VHOST_GET_VRING_BASE, argp);
+}
+
+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_SET_STATE:
+		r = vhost_set_state(m, argp);
+		break;
+	case VHOST_GET_FEATURES:
+		r = vhost_get_features(m, argp);
+		break;
+	case VHOST_SET_FEATURES:
+		r = vhost_set_features(m, argp);
+		break;
+	case VHOST_GET_VRING_BASE:
+		r = vhost_get_vring_base(m, argp);
+		break;


Does it mean the SET_VRING_BASE may only take affect after 
VHOST_MEV_SET_STATE?

+	default:
+		r = vhost_dev_ioctl(&m->dev, cmd, argp);
+		if (r == -ENOIOCTLCMD)
+			r = vhost_vring_ioctl(&m->dev, 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;
+
+	m = kzalloc(sizeof(*m), GFP_KERNEL | __GFP_RETRY_MAYFAIL);
+	if (!m)
+		return -ENOMEM;
+
+	mutex_init(&m->mutex);
+
+	nvqs = ops->get_queue_max(mdev);
+	m->nvqs = nvqs;


The name could be confusing, get_queue_max() is to get the maximum 
number of entries for a virtqueue supported by this device.

It looks to me that we need another API to query the maximum number of 
virtqueues supported by the device.

Thanks

quoted hunk ↗ jump to hunk

+
+	m->vqs = kmalloc_array(nvqs, sizeof(struct vhost_virtqueue),
+			       GFP_KERNEL);
+	if (!m->vqs) {
+		r = -ENOMEM;
+		goto err;
+	}
+
+	r = vfio_add_group_dev(dev, &vfio_vhost_mdev_dev_ops, m);
+	if (r)
+		goto err;
+
+	m->features = ops->get_features(mdev);
+	m->mdev = mdev;
+	return 0;
+
+err:
+	kfree(m->vqs);
+	kfree(m);
+	return r;
+}
+
+static void vhost_mdev_remove(struct device *dev)
+{
+	struct vhost_mdev *m;
+
+	m = vfio_del_group_dev(dev);
+	mutex_destroy(&m->mutex);
+	kfree(m->vqs);
+	kfree(m);
+}
+
+static struct mdev_class_id id_table[] = {
+	{ MDEV_ID_VHOST },
+	{ 0 },
+};
+
+static struct mdev_driver vhost_mdev_driver = {
+	.name	= "vhost_mdev",
+	.probe	= vhost_mdev_probe,
+	.remove	= vhost_mdev_remove,
+	.id_table = id_table,
+};
+
+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/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index 40d028eed645..5afbc2f08fa3 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h

@@ -116,4 +116,12 @@
  #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 */
+
+#define VHOST_MDEV_SET_STATE	_IOW(VHOST_VIRTIO, 0x70, __u64)
+
+#define VHOST_MDEV_S_STOPPED	0
+#define VHOST_MDEV_S_RUNNING	1
+#define VHOST_MDEV_S_MAX	2
+
  #endif

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