Re: [Qemu-devel] [PATCH 3/7] vfio: Check guest IOVA ranges against host IOMMU capabilities

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

On Thu, 2015-09-24 at 14:33 +1000, David Gibson wrote:
> The current vfio core code assumes that the host IOMMU is capable of
> mapping any IOVA the guest wants to use to where we need.  However, real
> IOMMUs generally only support translating a certain range of IOVAs (the
> "DMA window") not a full 64-bit address space.
> 
> The common x86 IOMMUs support a wide enough range that guests are very
> unlikely to go beyond it in practice, however the IOMMU used on IBM Power
> machines - in the default configuration - supports only a much more limited
> IOVA range, usually 0..2GiB.
> 
> If the guest attempts to set up an IOVA range that the host IOMMU can't
> map, qemu won't report an error until it actually attempts to map a bad
> IOVA.  If guest RAM is being mapped directly into the IOMMU (i.e. no guest
> visible IOMMU) then this will show up very quickly.  If there is a guest
> visible IOMMU, however, the problem might not show up until much later when
> the guest actually attempt to DMA with an IOVA the host can't handle.
> 
> This patch adds a test so that we will detect earlier if the guest is
> attempting to use IOVA ranges that the host IOMMU won't be able to deal
> with.
> 
> For now, we assume that "Type1" (x86) IOMMUs can support any IOVA, this is
> incorrect, but no worse than what we have already.  We can't do better for
> now because the Type1 kernel interface doesn't tell us what IOVA range the
> IOMMU actually supports.
> 
> For the Power "sPAPR TCE" IOMMU, however, we can retrieve the supported
> IOVA range and validate guest IOVA ranges against it, and this patch does
> so.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> Reviewed-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  hw/vfio/common.c              | 40 +++++++++++++++++++++++++++++++++++++---
>  include/hw/vfio/vfio-common.h |  6 ++++++
>  2 files changed, 43 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index 95a4850..f90cc75 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -343,14 +343,22 @@ static void vfio_listener_region_add(MemoryListener *listener,
>      if (int128_ge(int128_make64(iova), llend)) {
>          return;
>      }
> +    end = int128_get64(llend);
> +
> +    if ((iova < container->min_iova) || ((end - 1) > container->max_iova)) {
> +        error_report("vfio: IOMMU container %p can't map guest IOVA region"
> +                     " 0x%"HWADDR_PRIx"..0x%"HWADDR_PRIx,
> +                     container, iova, end - 1);
> +        ret = -EFAULT; /* FIXME: better choice here? */

"Bad address" makes sense to me.  This looks like an RFC comment, can we
remove it?

> +        goto fail;
> +    }
>  
>      memory_region_ref(section->mr);
>  
>      if (memory_region_is_iommu(section->mr)) {
>          VFIOGuestIOMMU *giommu;
>  
> -        trace_vfio_listener_region_add_iommu(iova,
> -                    int128_get64(int128_sub(llend, int128_one())));
> +        trace_vfio_listener_region_add_iommu(iova, end - 1);
>          /*
>           * FIXME: We should do some checking to see if the
>           * capabilities of the host VFIO IOMMU are adequate to model
> @@ -387,7 +395,6 @@ static void vfio_listener_region_add(MemoryListener *listener,
>  
>      /* Here we assume that memory_region_is_ram(section->mr)==true */
>  
> -    end = int128_get64(llend);
>      vaddr = memory_region_get_ram_ptr(section->mr) +
>              section->offset_within_region +
>              (iova - section->offset_within_address_space);
> @@ -685,7 +692,19 @@ static int vfio_connect_container(VFIOGroup *group, AddressSpace *as)
>              ret = -errno;
>              goto free_container_exit;
>          }
> +
> +        /*
> +         * FIXME: This assumes that a Type1 IOMMU can map any 64-bit
> +         * IOVA whatsoever.  That's not actually true, but the current
> +         * kernel interface doesn't tell us what it can map, and the
> +         * existing Type1 IOMMUs generally support any IOVA we're
> +         * going to actually try in practice.
> +         */
> +        container->min_iova = 0;
> +        container->max_iova = (hwaddr)-1;
>      } else if (ioctl(fd, VFIO_CHECK_EXTENSION, VFIO_SPAPR_TCE_IOMMU)) {
> +        struct vfio_iommu_spapr_tce_info info;
> +
>          ret = ioctl(group->fd, VFIO_GROUP_SET_CONTAINER, &fd);
>          if (ret) {
>              error_report("vfio: failed to set group container: %m");
> @@ -710,6 +729,21 @@ static int vfio_connect_container(VFIOGroup *group, AddressSpace *as)
>              ret = -errno;
>              goto free_container_exit;
>          }
> +
> +        /*
> +         * FIXME: This only considers the host IOMMU' 32-bit window.

IOMMU's?

> +         * At some point we need to add support for the optional
> +         * 64-bit window and dynamic windows
> +         */
> +        info.argsz = sizeof(info);
> +        ret = ioctl(fd, VFIO_IOMMU_SPAPR_TCE_GET_INFO, &info);
> +        if (ret) {
> +            error_report("vfio: VFIO_IOMMU_SPAPR_TCE_GET_INFO failed: %m");
> +            ret = -errno;
> +            goto free_container_exit;
> +        }
> +        container->min_iova = info.dma32_window_start;
> +        container->max_iova = container->min_iova + info.dma32_window_size - 1;
>      } else {
>          error_report("vfio: No available IOMMU models");
>          ret = -EINVAL;
> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
> index fbbe6de..859dbec 100644
> --- a/include/hw/vfio/vfio-common.h
> +++ b/include/hw/vfio/vfio-common.h
> @@ -65,6 +65,12 @@ typedef struct VFIOContainer {
>      MemoryListener listener;
>      int error;
>      bool initialized;
> +    /*
> +     * FIXME: This assumes the host IOMMU can support only a
> +     * single contiguous IOVA window.  We may need to generalize
> +     * that in future
> +     */

There sure are a lot of FIXMEs here.  This just seems to be an
implementation note.  I certainly encourage comments, but they don't all
need to start with FIXME unless it's something we really should fix.
"... may need to generalize..." does not sound like such a case.

> +    hwaddr min_iova, max_iova;
>      QLIST_HEAD(, VFIOGuestIOMMU) giommu_list;
>      QLIST_HEAD(, VFIOGroup) group_list;
>      QLIST_ENTRY(VFIOContainer) next;