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

[Bharat Bhushan]

Hi Eric,

On Fri, Apr 24, 2020 at 7:47 PM Auger Eric <address@hidden> wrote:
>
> Hi Bharat,
>
> On 4/2/20 11:01 AM, Bharat Bhushan wrote:
> > Hi Eric/Alex,
> >
> >> -----Original Message-----
> >> From: Alex Williamson <address@hidden>
> >> Sent: Thursday, March 26, 2020 11:23 PM
> >> To: Auger Eric <address@hidden>
> >> Cc: Bharat Bhushan <address@hidden>; address@hidden;
> >> address@hidden; address@hidden; address@hidden;
> >> address@hidden; Tomasz Nowicki [C] <address@hidden>;
> >> address@hidden; address@hidden; address@hidden; qemu-
> >> address@hidden; address@hidden; address@hidden;
> >> address@hidden; David Gibson <address@hidden>
> >> Subject: [EXT] Re: [PATCH v9 1/9] hw/vfio/common: Remove error print on 
> >> mmio
> >> region translation by viommu
> >>
> >> External Email
> >>
> >> ----------------------------------------------------------------------
> >> On Thu, 26 Mar 2020 18:35:48 +0100
> >> Auger Eric <address@hidden> 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 <address@hidden> 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 <address@hidden>
> >>>>> Signed-off-by: Bharat Bhushan <address@hidden>
> >>>>> ---
> >>>>>  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,
> >
> > Let me know if I understood the proposal correctly:
> >
> > virtio-iommu driver in guest will make map (VIRTIO_IOMMU_T_MAP) with 
> > VIRTIO_IOMMU_MAP_F_MMIO flag for MSI mapping.
> > In qemu, virtio-iommu device will set a new defined flag (say IOMMU_MMIO) 
> > in iotlb->perm in memory_region_notify_iommu(). vfio_get_vaddr() will check 
> > same flag and will not print the warning.>
> > Is above correct?
> Yes that's what I had in mind.

In that case virtio-iommu driver in guest should not make map
(VIRTIO_IOMMU_T_MAP) call as it known nothing to be mapped.

Stay Safe

Thanks
-Bharat

>
> Thanks
>
> Eric
> >
> > Thanks
> > -Bharat
> >
> >>
> >> Alex
> >
> >
>