linux-kernel - Re: [RFC v3 4/8] vfio/type1: Add VFIO_NESTING_GET_IOMMU_UAPI

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Date:   Wed, 29 Jan 2020 16:56:49 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>
Cc:     eric.auger@...hat.com, kevin.tian@...el.com,
        jacob.jun.pan@...ux.intel.com, joro@...tes.org,
        ashok.raj@...el.com, jun.j.tian@...el.com, yi.y.sun@...el.com,
        jean-philippe.brucker@....com, peterx@...hat.com,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
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@...el.com> wrote:

> From: Liu Yi L <yi.l.liu@...el.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@...el.com>
> CC: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Cc: Alex Williamson <alex.williamson@...hat.com>
> Cc: Eric Auger <eric.auger@...hat.com>
> Cc: Jean-Philippe Brucker <jean-philippe.brucker@....com>
> Signed-off-by: Liu Yi L <yi.l.liu@...el.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.
Thanks,

Alex

>  	case VFIO_CHECK_EXTENSION:
>  		ret = vfio_ioctl_check_extension(container, arg);
>  		break;
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index d4bf415..62113be 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -857,6 +857,16 @@ struct vfio_iommu_type1_pasid_quota {
>   */
>  #define VFIO_IOMMU_SET_PASID_QUOTA	_IO(VFIO_TYPE, VFIO_BASE + 23)
>  
> +/**
> + * VFIO_NESTING_GET_IOMMU_UAPI_VERSION - _IO(VFIO_TYPE, VFIO_BASE + 24)
> + *
> + * Report the version of the IOMMU UAPI when dual stage IOMMU is supported.
> + * In VFIO, it is needed for VFIO_TYPE1_NESTING_IOMMU.
> + * Availability: Always.
> + * Return: IOMMU UAPI version
> + */
> +#define VFIO_NESTING_GET_IOMMU_UAPI_VERSION	_IO(VFIO_TYPE, VFIO_BASE + 24)
> +
>  /* -------- Additional API for SPAPR TCE (Server POWERPC) IOMMU -------- */
>  
>  /*