Re: [RFC v3 4/8] vfio/type1: Add VFIO_NESTING_GET_IOMMU_UAPI_VERSION

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 31 Jan 2020 13:04:11 +0000
"Liu, Yi L" <yi.l.liu@intel.com> wrote:

> Hi Alex,
> 
> > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > Sent: Thursday, January 30, 2020 7:57 AM
> > To: Liu, Yi L <yi.l.liu@intel.com>
> > Subject: Re: [RFC v3 4/8] vfio/type1: Add
> > VFIO_NESTING_GET_IOMMU_UAPI_VERSION
> > 
> > On Wed, 29 Jan 2020 04:11:48 -0800
> > "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> >   
> > > From: Liu Yi L <yi.l.liu@intel.com>
> > >
> > > In Linux Kernel, the IOMMU nesting translation (a.k.a. IOMMU dual stage
> > > translation capability) is abstracted in uapi/iommu.h, in which the uAPIs
> > > like bind_gpasid/iommu_cache_invalidate/fault_report/pgreq_resp are defined.
> > >
> > > VFIO_TYPE1_NESTING_IOMMU stands for the vfio iommu type which is backed by
> > > IOMMU nesting translation capability. VFIO exposes the nesting capability
> > > to userspace and also exposes uAPIs (will be added in later patches) to user
> > > space for setting up nesting translation from userspace. Thus applications
> > > like QEMU could support vIOMMU for pass-through devices with IOMMU nesting
> > > translation capability.
> > >
> > > As VFIO expose the nesting IOMMU programming to userspace, it also needs to
> > > provide an API for the uapi/iommu.h version check to ensure compatibility.
> > > This patch reports the iommu uapi version to userspace. Applications could
> > > use this API to do version check before further using the nesting uAPIs.
> > >
> > > Cc: Kevin Tian <kevin.tian@intel.com>
> > > CC: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > Cc: Alex Williamson <alex.williamson@redhat.com>
> > > Cc: Eric Auger <eric.auger@redhat.com>
> > > Cc: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> > > Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> > > ---
> > >  drivers/vfio/vfio.c       |  3 +++
> > >  include/uapi/linux/vfio.h | 10 ++++++++++
> > >  2 files changed, 13 insertions(+)
> > >
> > > diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> > > index 425d60a..9087ad4 100644
> > > --- a/drivers/vfio/vfio.c
> > > +++ b/drivers/vfio/vfio.c
> > > @@ -1170,6 +1170,9 @@ static long vfio_fops_unl_ioctl(struct file *filep,
> > >  	case VFIO_GET_API_VERSION:
> > >  		ret = VFIO_API_VERSION;
> > >  		break;
> > > +	case VFIO_NESTING_GET_IOMMU_UAPI_VERSION:
> > > +		ret = iommu_get_uapi_version();
> > > +		break;  
> > 
> > Shouldn't the type1 backend report this?  It doesn't make much sense
> > that the spapr backend reports a version for something it doesn't
> > support.  Better yet, provide this info gratuitously in the
> > VFIO_IOMMU_GET_INFO ioctl return like you do with nesting in the next
> > patch, then it can help the user figure out if this support is present.  
> 
> yeah, it would be better to report it by type1 backed. However,
> it is kind of issue when QEMU using it.
> 
> My series "hooks" vSVA supports on VFIO_TYPE1_NESTING_IOMMU type.
> [RFC v3 09/25] vfio: check VFIO_TYPE1_NESTING_IOMMU support
> https://www.spinics.net/lists/kvm/msg205197.html
> 
> In QEMU, it will determine the iommu type firstly and then invoke
> VFIO_SET_IOMMU. I think before selecting VFIO_TYPE1_NESTING_IOMMU,
> QEMU needs to check the IOMMU uAPI version. If IOMMU uAPI is incompatible,
> QEMU should not use VFIO_TYPE1_NESTING_IOMMU type. If
> VFIO_NESTING_GET_IOMMU_UAPI_VERSION is available after set iommu, then it
> may be an issue. That's why this series reports the version in vfio layer
> instead of type1 backend.

Why wouldn't you use CHECK_EXTENSION?  You could probe specifically for
a VFIO_TYP1_NESTING_IOMMU_UAPI_VERSION extension that returns the
version number.  Thanks,

Alex