linux-kernel - RE: [PATCH v1 7/8] vfio/type1: Add VFIO_IOMMU_CACHE

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Date:   Thu, 16 Apr 2020 10:40:03 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
CC:     "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>,
        "Sun, Yi Y" <yi.y.sun@...el.com>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Wu, Hao" <hao.wu@...el.com>
Subject: RE: [PATCH v1 7/8] vfio/type1: Add VFIO_IOMMU_CACHE_INVALIDATE

Hi Alex,
Still have a direction question with you. Better get agreement with you
before heading forward.

> From: Alex Williamson <alex.williamson@...hat.com>
> 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
 
Thanks,
Yi Liu