Re: [kvm-devel] [PATCH][RFC] KVM: prepare user interface for smp guests

[Arnd Bergmann <arnd@arndb.de>]

On Sunday 29 October 2006 14:31, Avi Kivity wrote:
> +       r = -EEXIST;
> +       if (vcpu->vmcs)
> +               goto out_unlock;
> +
> +       r = -ENOMEM;
> +       filp = get_empty_filp();
> +       if (!filp)
> +               goto out_unlock;
> +
> +       r = get_unused_fd();
> +       if (r < 0)
> +               goto out_free_filp;
> +
> +       fd = r;
>  
>         vcpu->host_fx_image = (char*)ALIGN((hva_t)vcpu->fx_buf,
>                                            FX_IMAGE_ALIGN);
> @@ -1372,10 +1428,25 @@ static int kvm_dev_ioctl_create_vcpu(str
>         if (r < 0)
>                 goto out_free_vcpus;
>  
> -       return 0;
> +       filp->f_dentry = dget(kvm_filp->f_dentry);
> +       filp->f_vfsmnt = mntget(kvm_filp->f_vfsmnt);
> +       filp->f_mode = kvm_filp->f_mode;
> +       allow_write_access(filp);
> +       cdev_get(filp->f_dentry->d_inode->i_cdev);
> +       kvm_get(kvm);
> +       filp->f_op = fops_get(&kvm_vcpu_ops);
> +       filp->private_data = vcpu;
> +       fd_install(fd, filp);

Separating the objects into different file descriptors sounds like a
good idea, but reusing an open dentry/inode with a new file and different
file operations is a rather unusual way to do it. Your concept of allocating
a new context on each open is already weird, but there have been other
examples of that before.

I'd suggest going to a syscall-based model with your own file system right
away, even if you don't use the spufs approach but something in the middle:

* You do a trivial nonmountable new file system with anonymous objects,
  similar to eventpollfs, and hand out file descriptors to inodes in it,
  for both the kvm and the vcpu objects.
* You replace the syscall you'd normally use to hand out a new kvm instance
  with an ioctl on /dev/kvm, and don't allow any other operations on that
  device.

This would be a much more consistant object model, compared with other
generic kernel functionality that is not bound to an actual device.
You still have all the flexibility of a loadable module without core
kernel changes for the development phase, and can easily switch to real
syscalls when merging it into mainline.

I really think that a small number of syscalls is where you should be
heading, whether you use a file system or not, but I understand that
ioctls are convenient for development.

	Arnd <><