linux-kernel - RE: [PATCH v4 04/15] vfio/type1: Report iommu nesting info to userspace

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Date:   Thu, 9 Jul 2020 00:25:24 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Auger Eric <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>,
        "stefanha@...il.com" <stefanha@...il.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 v4 04/15] vfio/type1: Report iommu nesting info to
 userspace

Hi Alex,

> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Thursday, July 9, 2020 3:30 AM
> 
> On Wed, 8 Jul 2020 08:08:40 +0000
> "Liu, Yi L" <yi.l.liu@...el.com> wrote:
> 
> > Hi Alex,
> >
> > Eric asked if we will to have data strcut other than struct iommu_nesting_info
> > type in the struct vfio_iommu_type1_info_cap_nesting @info[] field. I'm not
> > quit sure on it. I guess the answer may be not as VFIO's nesting support should
> > based on IOMMU UAPI. how about your opinion?
> >
> > +#define VFIO_IOMMU_TYPE1_INFO_CAP_NESTING  3
> > +
> > +/*
> > + * 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[];
> > +};
> 
> It's not a very useful uAPI if the user can't be sure what they're
> getting out of it.  Info capabilities are "cheap", they don't need to
> be as extensible as an ioctl.  It's not clear that we really even need
> the flags (and therefore the padding), just define it to return the
> IOMMU uAPI structure with no extensibility.  If we need to expose
> something else, create a new capability.  Thanks,

thanks for the guiding, then I may embed the struct iommu_nesting_info
here. :-)

Regards,
Yi Liu

> Alex
> 
> >
> > https://lore.kernel.org/linux-
> iommu/DM5PR11MB1435290B6CD561EC61027892C3690@...PR11MB1435.nam
> prd11.prod.outlook.com/
> >
> > Regards,
> > Yi Liu
> >
> > > From: Liu, Yi L
> > > Sent: Tuesday, July 7, 2020 5:32 PM
> > >
> > [...]
> > > > >
> > > > >>> +
> > > > >>> +/*
> > > > >>> + * 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.
> > > > >> Is it expected to change?
> > > > >
> > > > > honestly, I'm not quite sure on it. I did considered to embed struct
> > > > > iommu_nesting_info here instead of using info[]. but I hesitated as
> > > > > using info[] may leave more flexibility on this struct. how about
> > > > > your opinion? perhaps it's fine to embed the struct
> > > > > iommu_nesting_info here as long as VFIO is setup nesting based on
> > > > > IOMMU UAPI.
> > > > >
> > > > >>> + */
> > > > >>> +struct vfio_iommu_type1_info_cap_nesting {
> > > > >>> +	struct	vfio_info_cap_header header;
> > > > >>> +	__u32	flags;
> > > > >> You may document flags.
> > > > >
> > > > > sure. it's reserved for future.
> > > > >
> > > > > Regards,
> > > > > Yi Liu
> > > > >
> > > > >>> +	__u32	padding;
> > > > >>> +	__u8	info[];
> > > > >>> +};
> > > > >>> +
> > > > >>>  #define VFIO_IOMMU_GET_INFO _IO(VFIO_TYPE, VFIO_BASE + 12)
> > > > >>>
> > > > >>>  /**
> > > > >>>
> > > > >> Thanks
> > > > >>
> > > > >> Eric
> > > > >
> >