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

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

On Thu, 26 Mar 2020 18:35:48 +0100
Auger Eric <eric.auger@redhat.com> wrote:

> Hi Alex,
> 
> On 3/24/20 12:08 AM, Alex Williamson wrote:
> > [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.  
> Yes exactly. The guest creates a mapping between one giova and this gpa
> (corresponding to the MSI controller doorbell) because MSIs are mapped
> on ARM. But practically the physical device is programmed with an host
> chosen iova that maps onto the physical MSI controller's doorbell. so
> the device never performs DMA accesses 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.  
> yes makes sense.
> 
>   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.  
> MSI doorbells are mapped using the IOMMU_MMIO flag (dma-iommu.c
> iommu_dma_get_msi_page).
> One idea could be to pass that flag through the IOMMU Notifier mechanism
> into the iotlb->perm. Eventually when we get this in vfio_get_vaddr() we
> would not print the warning. Could that make sense?

Yeah, if we can identify a valid case that doesn't need a warning,
that's fine by me.  Thanks,

Alex