Re: [Qemu-devel] [PATCH] vfio: Filter out bogus mappings

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jan 15, 2014 at 12:44:08PM -0700, Alex Williamson wrote:
> Since 57271d63 we now see spurious mappings with the upper bits set
> if 64bit PCI BARs are sized while enabled.  The guest writes a mask
> of 0xffffffff to the lower BAR to size it, then restores it, then
> writes the same mask to the upper BAR resulting in a spurious BAR
> mapping into the last 4G of the 64bit address space.  Most
> architectures do not support or make use of the full 64bits address
> space for PCI BARs, so we filter out mappings with the high bit set.
> Long term, we probably need to think about vfio telling us the
> address width limitations of the IOMMU.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  hw/misc/vfio.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/misc/vfio.c b/hw/misc/vfio.c
> index 30b1a78..c8f63a6 100644
> --- a/hw/misc/vfio.c
> +++ b/hw/misc/vfio.c
> @@ -2156,7 +2156,9 @@ static int vfio_dma_map(VFIOContainer *container, hwaddr iova,
>  
>  static bool vfio_listener_skipped_section(MemoryRegionSection *section)
>  {
> -    return !memory_region_is_ram(section->mr);
> +    return !memory_region_is_ram(section->mr) ||
> +             section->offset_within_address_space >
> +             ~section->offset_within_address_space;

I personally find this a very un-intuitive way to test it.

 section->offset_within_address_space >= 0x1ULL << 63

would be more readable I think.

A comment in code explaining what this hack does won't hurt too.

>  }
>  
>  static void vfio_listener_region_add(MemoryListener *listener,