linux-kernel - RE: [PATCH v3 03/14] vfio/type1: Report iommu nesting info to userspace

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Date:   Fri, 3 Jul 2020 13:03:17 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "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>,
        "Wu, Hao" <hao.wu@...el.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>
Subject: RE: [PATCH v3 03/14] vfio/type1: Report iommu nesting info to
 userspace

Hi Alex,

> From: Liu, Yi L <yi.l.liu@...el.com>
> Sent: Friday, July 3, 2020 2:06 PM
[...]
> > > +#define VFIO_IOMMU_TYPE1_INFO_CAP_NESTING  3
> > > +
> > > +struct vfio_iommu_type1_info_cap_nesting {
> > > +	struct	vfio_info_cap_header header;
> > > +	__u32	flags;
> >
> > I think there's an alignment issue here for a uapi.  The header field
> > is 8-bytes total and info[] should start at an 8-byte alignment to
> > allow data[] within info to have 8-byte alignment.  This could lead to
> > the structure having a compiler dependent size and offsets.  We should
> > add a 4-byte reserved field here to resolve.
> 
> got it. or how about defining the flags as __u64?
> 
> >
> > > +	__u8	info[];
> > > +};
> >
> > This should have a lot more description around it, a user could not
> > infer that info[] is including a struct iommu_nesting_info from the
> > information provided here.
> > Thanks,
> 
> sure. BTW. do you think it is necessary to add a flag to indicate the info[] is a
> struct iommu_nesting_info? or as a start, it's not necessary to do it.

seems like I misunderstood your comment. Does below description suits
your comment?

/*
 * Reporting nesting info to user space.
 *
 * @info:       the nesting info provided by IOMMU driver. Today
 *              it is expected to be a struct iommu_nesting_info
 *              data.
 */
struct vfio_iommu_type1_info_cap_nesting {
        struct  vfio_info_cap_header header;
        __u32   flags;
        __u32   padding;
        __u8    info[];
};

Thanks,
Yi Liu

> Regards,
> Yi Liu
> 
> > Alex
> >
> > > +
> > >  #define VFIO_IOMMU_GET_INFO _IO(VFIO_TYPE, VFIO_BASE + 12)
> > >
> > >  /**