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

[Liu, Yi L]

> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Tuesday, February 4, 2020 2:01 AM
> To: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> Subject: Re: [RFC v3 4/8] vfio/type1: Add
> VFIO_NESTING_GET_IOMMU_UAPI_VERSION
> 
> On Fri, 31 Jan 2020 13:04:11 +0000
> "Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote:
> 
> > Hi Alex,
> >
> > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> > > Sent: Thursday, January 30, 2020 7:57 AM
> > > To: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > > 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@xxxxxxxxx> wrote:
> > >
> > > > From: Liu Yi L <yi.l.liu@xxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxx>
> > > > CC: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > > > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > > Cc: Eric Auger <eric.auger@xxxxxxxxxx>
> > > > Cc: Jean-Philippe Brucker <jean-philippe.brucker@xxxxxxx>
> > > > Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
> > > > ---
> > > >  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,

oh, yes. Thanks for this guiding. :-)

Regards,
Yi Liu