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

[Bharat Bhushan]

hi Eric,

On Tue, May 5, 2020 at 3:00 PM Auger Eric <address@hidden> wrote:
>
> Hi Bharat,
>
> On 5/5/20 11:25 AM, Bharat Bhushan wrote:
> > 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.
> sorry I don't catch what you meant. Please can you elaborate?

What I understood of the proposal is:
Linux:
 1) MSI doorbells are mapped using the IOMMU_MMIO flag (dma-iommu.c
iommu_dma_get_msi_page)
 2) virtio-iommu driver in guest will make map (VIRTIO_IOMMU_T_MAP)
with VIRTIO_IOMMU_MAP_F_MMIO flag for MSI mapping.

GEMU:
3) virtio-iommu device - If VIRTIO_IOMMU_MAP_F_MMIO flag set then will
set a new defined flag (say IOMMU_MMIO) in iotlb->perm in
memory_region_notify_iommu()
4. vfio_get_vaddr() will check same flag and will not print the
warning. Also vfio_iommu_map_notify() will not do anything.

So, rather than going down to step 3 and 4, can we avoid maling map()
calling in step-2 itself?

Thanks
-Bharat

>
> Thanks
>
> Eric
> >
> > Stay Safe
> >
> > Thanks
> > -Bharat
> >
> >>
> >> Thanks
> >>
> >> Eric
> >>>
> >>> Thanks
> >>> -Bharat
> >>>
> >>>>
> >>>> Alex
> >>>
> >>>
> >>
> >
>