RE: [PATCH v4 6/9] iommufd: Reserved -1 in the iommufd xarray

Tian, Kevin
2023-04-27T06:42:03Z

> From: Liu, Yi L <yi.l.liu@intel.com>
> Sent: Wednesday, April 26, 2023 10:54 PM
>
> VFIO needs two reserved values. 0 is already reserved by initializing
> xarray with XA_FLAGS_ALLOC1. This reserves -1 by limiting the xa alloc
> range.
>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> ---
> drivers/iommu/iommufd/main.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iommu/iommufd/main.c
> b/drivers/iommu/iommufd/main.c
> index 3fbe636c3d8a..51b27c96c52f 100644
> --- a/drivers/iommu/iommufd/main.c
> +++ b/drivers/iommu/iommufd/main.c
> @@ -28,6 +28,9 @@ struct iommufd_object_ops {
> static const struct iommufd_object_ops iommufd_object_ops[];
> static struct miscdevice vfio_misc_dev;
>
> +/* -1 is reserved */
> +#define iommufd_xa_limit_32b XA_LIMIT(0, (-2U))
> +
> struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
> size_t size,
> enum iommufd_object_type type)
> @@ -50,7 +53,7 @@ struct iommufd_object *_iommufd_object_alloc(struct
> iommufd_ctx *ictx,
> * before calling iommufd_object_finalize().
> */
> rc = xa_alloc(&ictx->objects, &obj->id, XA_ZERO_ENTRY,
> - xa_limit_32b, GFP_KERNEL_ACCOUNT);
> + iommufd_xa_limit_32b, GFP_KERNEL_ACCOUNT);

Just direct use XA_LIMIT() here.

btw do we need a contract so vfio can learn 0 and -1 are reserved or
fine to have a fixed assumption in later patches?