Re: [PATCH v9 1/9] hw/vfio/common: Remove error print on mmio region translation by viommu

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

[Cc +dwg who originated this warning]

On Mon, 23 Mar 2020 14:16:09 +0530
Bharat Bhushan <bbhushan2@marvell.com> wrote:

> On ARM, the MSI doorbell is translated by the virtual IOMMU.
> As such address_space_translate() returns the MSI controller
> MMIO region and we get an "iommu map to non memory area"
> message. Let's remove this latter.
> 
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> Signed-off-by: Bharat Bhushan <bbhushan2@marvell.com>
> ---
>  hw/vfio/common.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index 5ca11488d6..c586edf47a 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -426,8 +426,6 @@ static bool vfio_get_vaddr(IOMMUTLBEntry *iotlb, void **vaddr,
>                                   &xlat, &len, writable,
>                                   MEMTXATTRS_UNSPECIFIED);
>      if (!memory_region_is_ram(mr)) {
> -        error_report("iommu map to non memory area %"HWADDR_PRIx"",
> -                     xlat);
>          return false;
>      }
>  

I'm a bit confused here, I think we need more justification beyond "we
hit this warning and we don't want to because it's ok in this one
special case, therefore remove it".  I assume the special case is that
the device MSI address is managed via the SET_IRQS ioctl and therefore
we won't actually get DMAs to this range.  But I imagine the case that
was in mind when adding this warning was general peer-to-peer between
and assigned and emulated device.  Maybe there's an argument to be made
that such a p2p mapping might also be used in a non-vIOMMU case.  We
skip creating those mappings and drivers continue to work, maybe
because nobody attempts to do p2p DMA with the types of devices we
emulate, maybe because p2p DMA is not absolutely reliable on bare metal
and drivers test it before using it.  Anyway, I need a better argument
why this is ok.  Thanks,

Alex