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

Jason Gunthorpe via iommu
2021-09-22T12:55:33Z

On Wed, Sep 22, 2021 at 03:41:50AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Wednesday, September 22, 2021 1:47 AM
> >
> > 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
>
> We don't have strong preference here. Just follow what vfio does
> today. So Alex's opinion is appreciated here. 😊

This is a uAPI design, it should follow the current mainstream
thinking on how to build these things. There is a lot of old stuff in
vfio that doesn't match the modern thinking. IMHO.

> > TYPE1V2 seems like nonsense
>
> just in case other mapping protocols are introduced in the future

Well, we should never, ever do that. Allowing PPC and evrything else
to split in VFIO has created a compelte disaster in userspace. HW
specific extensions should be modeled as extensions not a wholesale
replacement of everything.

I'd say this is part of the modern thinking on uAPI design.

What I want to strive for is the basic API is usable with all HW - and
is what something like DPDK can exclusively use.

An extended API with HW specific facets exists for qemu to use to
build a HW backed accelereated and featureful vIOMMU emulation.

The needs of qmeu should not trump the requirement for a universal
basic API.

Eg if we can't figure out a basic API version of the PPC range issue
then that should be punted to a PPC specific API.

Jason