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SubjectRe: [PATCH 4/6] vhost_vdpa: support doorbell mapping via mmap
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Hi,

On 29.5.2020 11.03, Jason Wang 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.

Just wondering. I know very little about vdpa. But how is such a "sw
doorbell" monitored or observed, if no fault or wmexit etc.
Is there some kind of polling used?

> 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>
> ---
> drivers/vhost/vdpa.c | 59 ++++++++++++++++++++++++++++++++++++++++++++
> 1 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 @@
> #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>
> @@ -741,12 +742,70 @@ 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,
> };
>

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