Re: [Qemu-devel] [PATCH v5 03/18] vfio: allow to notify unmap for very large region

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

On Tue, 24 Jan 2017 18:25:56 +0800
Peter Xu <peterx@redhat.com> wrote:

> Linux vfio driver supports to do VFIO_IOMMU_UNMAP_DMA for a very big
> region. This can be leveraged by QEMU IOMMU implementation to cleanup
> existing page mappings for an entire iova address space (by notifying
> with an IOTLB with extremely huge addr_mask). However current
> vfio_iommu_map_notify() does not allow that. It make sure that all the
> translated address in IOTLB is falling into RAM range.
> 
> The check makes sense, but it should only be a sensible checker for
> mapping operations, and mean little for unmap operations.
> 
> This patch moves this check into map logic only, so that we'll get
> faster unmap handling (no need to translate again), and also we can then
> better support unmapping a very big region when it covers non-ram ranges
> or even not-existing ranges.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  hw/vfio/common.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index ce55dff..4d90844 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -354,11 +354,10 @@ static void vfio_iommu_map_notify(IOMMUNotifier *n, IOMMUTLBEntry *iotlb)
>          return;
>      }
>  
> -    if (!vfio_get_vaddr(iotlb, &vaddr, &read_only)) {
> -        return;
> -    }
> -
>      if ((iotlb->perm & IOMMU_RW) != IOMMU_NONE) {
> +        if (!vfio_get_vaddr(iotlb, &vaddr, &read_only)) {
> +            return;
> +        }


David, is SPAPR going to freak out if it sees unmaps to ranges that
extend beyond individual mappings, or perhaps include no mappings?
This effectively allows unmapping the entire address space of the iommu
in one pass, without validating or translating the backing.

>          ret = vfio_dma_map(container, iova,
>                             iotlb->addr_mask + 1, vaddr,
>                             read_only);