RE: [PATCH v1 7/8] vfio/type1: Add VFIO_IOMMU_CACHE_INVALIDATE

[Tian, Kevin]

> From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> Sent: Thursday, April 16, 2020 6:40 PM
> 
> Hi Alex,
> Still have a direction question with you. Better get agreement with you
> before heading forward.
> 
> > From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Sent: Friday, April 3, 2020 11:35 PM
> [...]
> > > > > + *
> > > > > + * returns: 0 on success, -errno on failure.
> > > > > + */
> > > > > +struct vfio_iommu_type1_cache_invalidate {
> > > > > +	__u32   argsz;
> > > > > +	__u32   flags;
> > > > > +	struct	iommu_cache_invalidate_info cache_info;
> > > > > +};
> > > > > +#define VFIO_IOMMU_CACHE_INVALIDATE      _IO(VFIO_TYPE,
> > VFIO_BASE
> > > > + 24)
> > > >
> > > > The future extension capabilities of this ioctl worry me, I wonder if
> > > > we should do another data[] with flag defining that data as
> CACHE_INFO.
> > >
> > > Can you elaborate? Does it mean with this way we don't rely on iommu
> > > driver to provide version_to_size conversion and instead we just pass
> > > data[] to iommu driver for further audit?
> >
> > No, my concern is that this ioctl has a single function, strictly tied
> > to the iommu uapi.  If we replace cache_info with data[] then we can
> > define a flag to specify that data[] is struct
> > iommu_cache_invalidate_info, and if we need to, a different flag to
> > identify data[] as something else.  For example if we get stuck
> > expanding cache_info to meet new demands and develop a new uapi to
> > solve that, how would we expand this ioctl to support it rather than
> > also create a new ioctl?  There's also a trade-off in making the ioctl
> > usage more difficult for the user.  I'd still expect the vfio layer to
> > check the flag and interpret data[] as indicated by the flag rather
> > than just passing a blob of opaque data to the iommu layer though.
> > Thanks,
> 
> Based on your comments about defining a single ioctl and a unified
> vfio structure (with a @data[] field) for pasid_alloc/free, bind/
> unbind_gpasid, cache_inv. After some offline trying, I think it would
> be good for bind/unbind_gpasid and cache_inv as both of them use the
> iommu uapi definition. While the pasid alloc/free operation doesn't.
> It would be weird to put all of them together. So pasid alloc/free
> may have a separate ioctl. It would look as below. Does this direction
> look good per your opinion?
> 
> ioctl #22: VFIO_IOMMU_PASID_REQUEST
> /**
>   * @pasid: used to return the pasid alloc result when flags == ALLOC_PASID
>   *         specify a pasid to be freed when flags == FREE_PASID
>   * @range: specify the allocation range when flags == ALLOC_PASID
>   */
> struct vfio_iommu_pasid_request {
> 	__u32	argsz;
> #define VFIO_IOMMU_ALLOC_PASID	(1 << 0)
> #define VFIO_IOMMU_FREE_PASID	(1 << 1)
> 	__u32	flags;
> 	__u32	pasid;
> 	struct {
> 		__u32	min;
> 		__u32	max;
> 	} range;
> };
> 
> ioctl #23: VFIO_IOMMU_NESTING_OP
> struct vfio_iommu_type1_nesting_op {
> 	__u32	argsz;
> 	__u32	flags;
> 	__u32	op;
> 	__u8	data[];
> };
> 
> /* Nesting Ops */
> #define VFIO_IOMMU_NESTING_OP_BIND_PGTBL        0
> #define VFIO_IOMMU_NESTING_OP_UNBIND_PGTBL      1
> #define VFIO_IOMMU_NESTING_OP_CACHE_INVLD       2
> 

Then why cannot we just put PASID into the header since the
majority of nested usage is associated with a pasid? 

ioctl #23: VFIO_IOMMU_NESTING_OP
struct vfio_iommu_type1_nesting_op {
	__u32	argsz;
	__u32	flags;
	__u32	op;
	__u32   pasid;
	__u8	data[];
};

In case of SMMUv2 which supports nested w/o PASID, this field can
be ignored for that specific case.

Thanks
Kevin