Re: [RFC 12/20] iommu/iommufd: Add IOMMU_CHECK_EXTENSION

Jason Gunthorpe via iommu
2021-09-21T17:47:22Z

On Sun, Sep 19, 2021 at 02:38:40PM +0800, Liu Yi L wrote:
> As aforementioned, userspace should check extension for what formats
> can be specified when allocating an IOASID. This patch adds such
> interface for userspace. In this RFC, iommufd reports EXT_MAP_TYPE1V2
> support and no no-snoop support yet.
>
> Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> drivers/iommu/iommufd/iommufd.c | 7 +++++++
> include/uapi/linux/iommu.h | 27 +++++++++++++++++++++++++++
> 2 files changed, 34 insertions(+)
>
> diff --git a/drivers/iommu/iommufd/iommufd.c b/drivers/iommu/iommufd/iommufd.c
> index 4839f128b24a..e45d76359e34 100644
> +++ b/drivers/iommu/iommufd/iommufd.c
> @@ -306,6 +306,13 @@ static long iommufd_fops_unl_ioctl(struct file *filep,
> return ret;
>
> switch (cmd) {
> + case IOMMU_CHECK_EXTENSION:
> + switch (arg) {
> + case EXT_MAP_TYPE1V2:
> + return 1;
> + default:
> + return 0;
> + }
> case IOMMU_DEVICE_GET_INFO:
> ret = iommufd_get_device_info(ictx, arg);
> break;
> diff --git a/include/uapi/linux/iommu.h b/include/uapi/linux/iommu.h
> index 5cbd300eb0ee..49731be71213 100644
> +++ b/include/uapi/linux/iommu.h
> @@ -14,6 +14,33 @@
> #define IOMMU_TYPE (';')
> #define IOMMU_BASE 100
>
> +/*
> + * IOMMU_CHECK_EXTENSION - _IO(IOMMU_TYPE, IOMMU_BASE + 0)
> + *
> + * Check whether an uAPI extension is supported.
> + *
> + * It's unlikely that all planned capabilities in IOMMU fd will be ready
> + * in one breath. User should check which uAPI extension is supported
> + * according to its intended usage.
> + *
> + * A rough list of possible extensions may include:
> + *
> + * - EXT_MAP_TYPE1V2 for vfio type1v2 map semantics;
> + * - EXT_DMA_NO_SNOOP for no-snoop DMA support;
> + * - EXT_MAP_NEWTYPE for an enhanced map semantics;
> + * - EXT_MULTIDEV_GROUP for 1:N iommu group;
> + * - EXT_IOASID_NESTING for what the name stands;
> + * - EXT_USER_PAGE_TABLE for user managed page table;
> + * - EXT_USER_PASID_TABLE for user managed PASID table;
> + * - EXT_DIRTY_TRACKING for tracking pages dirtied by DMA;
> + * - ...
> + *
> + * Return: 0 if not supported, 1 if supported.
> + */
> +#define EXT_MAP_TYPE1V2 1
> +#define EXT_DMA_NO_SNOOP 2
> +#define IOMMU_CHECK_EXTENSION _IO(IOMMU_TYPE, IOMMU_BASE + 0)

I generally advocate for a 'try and fail' approach to discovering
compatibility.

If that doesn't work for the userspace then a query to return a
generic capability flag is the next best idea. Each flag should
clearly define what 'try and fail' it is talking about

Eg dma_no_snoop is about creating an IOS with flag NO SNOOP set

TYPE1V2 seems like nonsense

Not sure about the others.

IOW, this should recast to a generic 'query capabilities' IOCTL

Jason