On Thu, 26 Mar 2020 18:35:48 +0100 Auger Eric <eric.au...@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 <bbhush...@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.au...@redhat.com> > >> Signed-off-by: Bharat Bhushan <bbhush...@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