Re: [PATCH 10/10] iommufd: Allow iommufd to supply /dev/vfio/vfio

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 25 Oct 2022 15:50:45 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> If the VFIO container is compiled out, give a kconfig option for iommufd
> to provide the miscdev node with the same name and permissions as vfio
> uses.
> 
> The compatibility node supports the same ioctls as VFIO and automatically
> enables the VFIO compatible pinned page accounting mode.

I think I'd like to see some sort of breadcrumb when /dev/vfio/vfio is
provided by something other than the vfio container code.  If we intend
to include this before P2P is resolved, that breadcrumb (dmesg I'm
guessing) might also list any known limitations of the compatibility to
save time with debugging.  Thanks,

Alex

> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/iommu/iommufd/Kconfig | 12 ++++++++++++
>  drivers/iommu/iommufd/main.c  | 35 ++++++++++++++++++++++++++++++++---
>  2 files changed, 44 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/iommufd/Kconfig b/drivers/iommu/iommufd/Kconfig
> index f0a2012234fa09..afc83b7575cce6 100644
> --- a/drivers/iommu/iommufd/Kconfig
> +++ b/drivers/iommu/iommufd/Kconfig
> @@ -14,6 +14,18 @@ config IOMMUFD
>  	  If you don't know what to do here, say N.
>  
>  if IOMMUFD
> +config IOMMUFD_VFIO_CONTAINER
> +	bool "IOMMUFD provides the VFIO container /dev/vfio/vfio"
> +	depends on VFIO && !VFIO_CONTAINER
> +	default VFIO && !VFIO_CONTAINER
> +	help
> +	  IOMMUFD will provide /dev/vfio/vfio instead of VFIO. This relies on
> +	  IOMMUFD providing compatibility emulation to give the same ioctls.
> +	  It provides an option to build a kernel with legacy VFIO components
> +	  removed.
> +
> +	  Unless testing IOMMUFD say N here.
> +
>  config IOMMUFD_TEST
>  	bool "IOMMU Userspace API Test support"
>  	depends on RUNTIME_TESTING_MENU
> diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c
> index 8a31c1a14cdd53..19db81fbf7f08f 100644
> --- a/drivers/iommu/iommufd/main.c
> +++ b/drivers/iommu/iommufd/main.c
> @@ -24,6 +24,7 @@
>  #include <uapi/linux/iommufd.h>
>  #include <linux/iommufd.h>
>  
> +#include "io_pagetable.h"
>  #include "iommufd_private.h"
>  #include "iommufd_test.h"
>  
> @@ -31,6 +32,7 @@ struct iommufd_object_ops {
>  	void (*destroy)(struct iommufd_object *obj);
>  };
>  static struct iommufd_object_ops iommufd_object_ops[];
> +static struct miscdevice vfio_misc_dev;
>  
>  struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
>  					     size_t size,
> @@ -167,6 +169,13 @@ static int iommufd_fops_open(struct inode *inode, struct file *filp)
>  	if (!ictx)
>  		return -ENOMEM;
>  
> +	/*
> +	 * For compatibility with VFIO when /dev/vfio/vfio is opened we default
> +	 * to the same rlimit accounting as vfio uses.
> +	 */
> +	if (filp->private_data == &vfio_misc_dev)
> +		ictx->account_mode = IOPT_PAGES_ACCOUNT_MM;
> +
>  	xa_init_flags(&ictx->objects, XA_FLAGS_ALLOC1 | XA_FLAGS_ACCOUNT);
>  	ictx->file = filp;
>  	filp->private_data = ictx;
> @@ -392,26 +401,46 @@ static struct miscdevice iommu_misc_dev = {
>  	.mode = 0660,
>  };
>  
> +
> +static struct miscdevice vfio_misc_dev = {
> +	.minor = VFIO_MINOR,
> +	.name = "vfio",
> +	.fops = &iommufd_fops,
> +	.nodename = "vfio/vfio",
> +	.mode = 0666,
> +};
> +
>  static int __init iommufd_init(void)
>  {
>  	int ret;
>  
>  	ret = misc_register(&iommu_misc_dev);
> -	if (ret) {
> -		pr_err("Failed to register misc device\n");
> +	if (ret)
>  		return ret;
> -	}
>  
> +	if (IS_ENABLED(CONFIG_IOMMUFD_VFIO_CONTAINER)) {
> +		ret = misc_register(&vfio_misc_dev);
> +		if (ret)
> +			goto err_misc;
> +	}
>  	return 0;
> +err_misc:
> +	misc_deregister(&iommu_misc_dev);
> +	return ret;
>  }
>  
>  static void __exit iommufd_exit(void)
>  {
> +	if (IS_ENABLED(CONFIG_IOMMUFD_VFIO_CONTAINER))
> +		misc_deregister(&vfio_misc_dev);
>  	misc_deregister(&iommu_misc_dev);
>  }
>  
>  module_init(iommufd_init);
>  module_exit(iommufd_exit);
>  
> +#if IS_ENABLED(CONFIG_IOMMUFD_VFIO_CONTAINER)
> +MODULE_ALIAS_MISCDEV(VFIO_MINOR);
> +#endif
>  MODULE_DESCRIPTION("I/O Address Space Management for passthrough devices");
>  MODULE_LICENSE("GPL");