Re: [PATCH v2 1/3] vfio: Find DMA available capability

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi Matthew,

On 9/15/20 12:29 AM, Matthew Rosato wrote:
> The underlying host may be limiting the number of outstanding DMA
> requests for type 1 IOMMU.  Add helper functions to check for the
> DMA available capability and retrieve the current number of DMA
> mappings allowed.
> 
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
>  hw/vfio/common.c              | 37 +++++++++++++++++++++++++++++++++++++
>  include/hw/vfio/vfio-common.h |  2 ++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index 3335714..7f4a338 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -844,6 +844,43 @@ vfio_get_region_info_cap(struct vfio_region_info *info, uint16_t id)
>      return NULL;
>  }
>  
> +static struct vfio_info_cap_header *
> +vfio_get_iommu_type1_info_cap(struct vfio_iommu_type1_info *info, uint16_t id)
> +{
> +    struct vfio_info_cap_header *hdr;
> +    void *ptr = info;
> +
> +    if (!(info->flags & VFIO_IOMMU_INFO_CAPS)) {
> +        return NULL;
> +    }
> +
> +    for (hdr = ptr + info->cap_offset; hdr != ptr; hdr = ptr + hdr->next) {
> +        if (hdr->id == id) {
> +            return hdr;
> +        }
> +    }
> +
> +    return NULL;
> +}
> +
> +bool vfio_get_info_dma_avail(struct vfio_iommu_type1_info *info,
> +                             unsigned int *avail)
> +{
> +    struct vfio_info_cap_header *hdr;
> +    struct vfio_iommu_type1_info_dma_avail *cap;
> +
> +    /* If the capability cannot be found, assume no DMA limiting */
> +    hdr = vfio_get_iommu_type1_info_cap(info,
> +                                        VFIO_IOMMU_TYPE1_INFO_DMA_AVAIL);
> +    if (hdr == NULL || avail == NULL) {

If you expect the caller to use avail=NULL, then why
return false when there is available information?

> +        return false;
> +    }
> +
> +    cap = (void *) hdr;
> +    *avail = cap->avail;
> +    return true;
> +}
> +
>  static int vfio_setup_region_sparse_mmaps(VFIORegion *region,
>                                            struct vfio_region_info *info)
>  {
> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
> index c78f3ff..661a380 100644
> --- a/include/hw/vfio/vfio-common.h
> +++ b/include/hw/vfio/vfio-common.h
> @@ -178,6 +178,8 @@ VFIOGroup *vfio_get_group(int groupid, AddressSpace *as, Error **errp);
>  void vfio_put_group(VFIOGroup *group);
>  int vfio_get_device(VFIOGroup *group, const char *name,
>                      VFIODevice *vbasedev, Error **errp);
> +bool vfio_get_info_dma_avail(struct vfio_iommu_type1_info *info,
> +                             unsigned int *avail);
>  
>  extern const MemoryRegionOps vfio_region_ops;
>  typedef QLIST_HEAD(VFIOGroupList, VFIOGroup) VFIOGroupList;
>