Re: [PATCH 4/6] vhost_vdpa: support doorbell mapping via mmap

[Rob Miller <rob.miller@broadcom.com>]

Given the need for 4K doorbell such that QEMU can easily map, ect, and assuming that I have a HW device which exposes 2 VQ's, with a notification area off of BAR3, offset=whatever, notifier_multiplier=4, we don't need to have 2 x 4K pages mapped into the VM for both doorbells do we? The guest driver would ring DB0 at BAR4+offset, and DB1 at BAR4+offset+(4*1).

The 4K per DB is usefulÂhow? This allows for QEMU trapping of individual DBs, that can then be used to do what, just forward the DBs via some other scheme - this makes sense for non-HW related Virtio devices I guess. Is this why there is a qemu option?

Rob Miller

rob.miller@broadcom.com

(919)721-3339

On Fri, May 29, 2020 at 4:03 AM Jason Wang <jasowang@redhat.com> wrote:

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>
---
Âdrivers/vhost/vdpa.c | 59 ++++++++++++++++++++++++++++++++++++++++++++
Â1 file changed, 59 insertions(+)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index 6ff72289f488..bbe23cea139a 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -15,6 +15,7 @@
Â#include <linux/module.h>
Â#include <linux/cdev.h>
Â#include <linux/device.h>
+#include <linux/mm.h>
Â#include <linux/iommu.h>
Â#include <linux/uuid.h>
Â#include <linux/vdpa.h>
@@ -741,12 +742,70 @@ static int vhost_vdpa_release(struct inode *inode, struct file *filep)
    return 0;
Â}

+static vm_fault_t vhost_vdpa_fault(struct vm_fault *vmf)
+{
+Â Â Â Âstruct vhost_vdpa *v = vmf->vma->vm_file->private_data;
+Â Â Â Âstruct vdpa_device *vdpa = v->vdpa;
+Â Â Â Âconst struct vdpa_config_ops *ops = vdpa->config;
+Â Â Â Âstruct vdpa_notification_area notify;
+Â Â Â Âstruct vm_area_struct *vma = vmf->vma;
+Â Â Â Âu16 index = vma->vm_pgoff;
+
+Â Â Â Ânotify = ops->get_vq_notification(vdpa, index);
+
+Â Â Â Âvma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+Â Â Â Âif (remap_pfn_range(vma, vmf->address & PAGE_MASK,
+Â Â Â Â Â Â Â Â Â Â Â Â Â Ânotify.addr >> PAGE_SHIFT, PAGE_SIZE,
+Â Â Â Â Â Â Â Â Â Â Â Â Â Âvma->vm_page_prot))
+Â Â Â Â Â Â Â Âreturn VM_FAULT_SIGBUS;
+
+Â Â Â Âreturn VM_FAULT_NOPAGE;
+}
+
+static const struct vm_operations_struct vhost_vdpa_vm_ops = {
+Â Â Â Â.fault = vhost_vdpa_fault,
+};
+
+static int vhost_vdpa_mmap(struct file *file, struct vm_area_struct *vma)
+{
+Â Â Â Âstruct vhost_vdpa *v = vma->vm_file->private_data;
+Â Â Â Âstruct vdpa_device *vdpa = v->vdpa;
+Â Â Â Âconst struct vdpa_config_ops *ops = vdpa->config;
+Â Â Â Âstruct vdpa_notification_area notify;
+Â Â Â Âint index = vma->vm_pgoff;
+
+Â Â Â Âif (vma->vm_end - vma->vm_start != PAGE_SIZE)
+Â Â Â Â Â Â Â Âreturn -EINVAL;
+Â Â Â Âif ((vma->vm_flags & VM_SHARED) == 0)
+Â Â Â Â Â Â Â Âreturn -EINVAL;
+Â Â Â Âif (vma->vm_flags & VM_READ)
+Â Â Â Â Â Â Â Âreturn -EINVAL;
+Â Â Â Âif (index > 65535)
+Â Â Â Â Â Â Â Âreturn -EINVAL;
+Â Â Â Âif (!ops->get_vq_notification)
+Â Â Â Â Â Â Â Âreturn -ENOTSUPP;
+
+Â Â Â Â/* To be safe and easily modelled by userspace, We only
+Â Â Â Â * support the doorbell which sits on the page boundary and
+Â Â Â Â * does not share the page with other registers.
+Â Â Â Â */
+Â Â Â Ânotify = ops->get_vq_notification(vdpa, index);
+Â Â Â Âif (notify.addr & (PAGE_SIZE - 1))
+Â Â Â Â Â Â Â Âreturn -EINVAL;
+Â Â Â Âif (vma->vm_end - vma->vm_start != notify.size)
+Â Â Â Â Â Â Â Âreturn -ENOTSUPP;
+
+Â Â Â Âvma->vm_ops = &vhost_vdpa_vm_ops;
+Â Â Â Âreturn 0;
+}
+
Âstatic const struct file_operations vhost_vdpa_fops = {
    .owner     = THIS_MODULE,
    .open     Â= vhost_vdpa_open,
    .release    = vhost_vdpa_release,
    .write_iter  Â= vhost_vdpa_chr_write_iter,
    .unlocked_ioctl = vhost_vdpa_unlocked_ioctl,
+   Â.mmap     Â= vhost_vdpa_mmap,
    .compat_ioctl Â= compat_ptr_ioctl,
Â};

--
2.20.1