netdev - Re: [RFC 2/4] vduse: Add file operation for mmap

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Message-ID: <CACGkMEuzrFP96qcFL0M=nGiQ9t57-EzOhZmB3No-8T8pMAWTxw@mail.gmail.com>
Date:   Wed, 28 Jun 2023 16:08:39 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Cindy Lu <lulu@...hat.com>
Cc:     mst@...hat.com, maxime.coquelin@...hat.com,
        xieyongji@...edance.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [RFC 2/4] vduse: Add file operation for mmap

On Wed, Jun 28, 2023 at 2:59 PM Cindy Lu <lulu@...hat.com> wrote:
>
> From: Your Name <you@...mple.com>
>
> Add the operation for mmap, The user space APP will
> use this function to map the pages to userspace

Please be specific in the log. E.g why and what the main goal for this mmap.

>
> Signed-off-by: Cindy Lu <lulu@...hat.com>
> ---
>  drivers/vdpa/vdpa_user/vduse_dev.c | 49 ++++++++++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
>
> diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> index f845dc46b1db..1b833bf0ae37 100644
> --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> @@ -1313,6 +1313,54 @@ static struct vduse_dev *vduse_dev_get_from_minor(int minor)
>         return dev;
>  }
>
> +
> +static vm_fault_t vduse_vm_fault(struct vm_fault *vmf)
> +{
> +       struct vduse_dev *dev = vmf->vma->vm_file->private_data;
> +       struct vm_area_struct *vma = vmf->vma;
> +       u16 index = vma->vm_pgoff;
> +
> +       struct vdpa_reconnect_info *info;
> +       info = &dev->reconnect_info[index];
> +
> +       vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +       if (remap_pfn_range(vma, vmf->address & PAGE_MASK, PFN_DOWN(info->addr),
> +                           PAGE_SIZE, vma->vm_page_prot))

I'm not sure if this can work e.g do we want to use separate pages for
each virtqueue (I think the answer is yes).

> +               return VM_FAULT_SIGBUS;
> +       return VM_FAULT_NOPAGE;
> +}
> +
> +static const struct vm_operations_struct vduse_vm_ops = {
> +       .fault = vduse_vm_fault,
> +};
> +
> +static int vduse_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +       struct vduse_dev *dev = file->private_data;
> +       struct vdpa_reconnect_info *info;
> +       unsigned long 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 (index > 65535)
> +               return -EINVAL;
> +
> +       info = &dev->reconnect_info[index];
> +       if (info->addr & (PAGE_SIZE - 1))
> +               return -EINVAL;
> +       if (vma->vm_end - vma->vm_start != info->size) {
> +               return -ENOTSUPP;
> +       }

How can userspace know the correct size (info->size) here?

> +
> +       vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);

Why do you need VM_IO, VM_PFNMAP and VM_DONTDUMP here?

Thanks

> +       vma->vm_ops = &vduse_vm_ops;
> +
> +       return 0;
> +}
> +
>  static int vduse_dev_open(struct inode *inode, struct file *file)
>  {
>         int ret;
> @@ -1345,6 +1393,7 @@ static const struct file_operations vduse_dev_fops = {
>         .unlocked_ioctl = vduse_dev_ioctl,
>         .compat_ioctl   = compat_ptr_ioctl,
>         .llseek         = noop_llseek,
> +       .mmap           = vduse_mmap,
>  };
>
>  static struct vduse_dev *vduse_dev_create(void)
> --
> 2.34.3
>