vhost_vdpa: support doorbell mapping via mmap

Currently the doorbell is relayed via eventfd which may have
significant overhead because of the cost of vmexits or syscall. This
patch introduces mmap() based doorbell mapping which can eliminate the
overhead caused by vmexit or syscall.

To ease the userspace modeling of the doorbell layout (usually
virtio-pci), this patch starts from a doorbell per page
model. Vhost-vdpa only support the hardware doorbell that sit at the
boundary of a page and does not share the page with other registers.

Doorbell of each virtqueue must be mapped separately, pgoff is the
index of the virtqueue. This allows userspace to map a subset of the
doorbell which may be useful for the implementation of software
assisted virtqueue (control vq) in the future.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200529080303.15449-5-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vdpa.c

@@ -15,6 +15,7 @@
 #include <linux/module.h>
 #include <linux/cdev.h>
 #include <linux/device.h>
+#include <linux/mm.h>
 #include <linux/iommu.h>
 #include <linux/uuid.h>
 #include <linux/vdpa.h>
@@ -739,12 +740,70 @@ static int vhost_vdpa_release(struct inode *inode, struct file *filep)
 	return 0;
 }
 
+static vm_fault_t vhost_vdpa_fault(struct vm_fault *vmf)
+{
+	struct vhost_vdpa *v = vmf->vma->vm_file->private_data;
+	struct vdpa_device *vdpa = v->vdpa;
+	const struct vdpa_config_ops *ops = vdpa->config;
+	struct vdpa_notification_area notify;
+	struct vm_area_struct *vma = vmf->vma;
+	u16 index = vma->vm_pgoff;
+
+	notify = ops->get_vq_notification(vdpa, index);
+
+	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+	if (remap_pfn_range(vma, vmf->address & PAGE_MASK,
+			    notify.addr >> PAGE_SHIFT, PAGE_SIZE,
+			    vma->vm_page_prot))
+		return VM_FAULT_SIGBUS;
+
+	return VM_FAULT_NOPAGE;
+}
+
+static const struct vm_operations_struct vhost_vdpa_vm_ops = {
+	.fault = vhost_vdpa_fault,
+};
+
+static int vhost_vdpa_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct vhost_vdpa *v = vma->vm_file->private_data;
+	struct vdpa_device *vdpa = v->vdpa;
+	const struct vdpa_config_ops *ops = vdpa->config;
+	struct vdpa_notification_area notify;
+	int index = vma->vm_pgoff;
+
+	if (vma->vm_end - vma->vm_start != PAGE_SIZE)
+		return -EINVAL;
+	if ((vma->vm_flags & VM_SHARED) == 0)
+		return -EINVAL;
+	if (vma->vm_flags & VM_READ)
+		return -EINVAL;
+	if (index > 65535)
+		return -EINVAL;
+	if (!ops->get_vq_notification)
+		return -ENOTSUPP;
+
+	/* To be safe and easily modelled by userspace, We only
+	 * support the doorbell which sits on the page boundary and
+	 * does not share the page with other registers.
+	 */
+	notify = ops->get_vq_notification(vdpa, index);
+	if (notify.addr & (PAGE_SIZE - 1))
+		return -EINVAL;
+	if (vma->vm_end - vma->vm_start != notify.size)
+		return -ENOTSUPP;
+
+	vma->vm_ops = &vhost_vdpa_vm_ops;
+	return 0;
+}
+
 static const struct file_operations vhost_vdpa_fops = {
 	.owner		= THIS_MODULE,
 	.open		= vhost_vdpa_open,
 	.release	= vhost_vdpa_release,
 	.write_iter	= vhost_vdpa_chr_write_iter,
 	.unlocked_ioctl	= vhost_vdpa_unlocked_ioctl,
+	.mmap		= vhost_vdpa_mmap,
 	.compat_ioctl	= compat_ptr_ioctl,
 };