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 3:16 PM Bharat Bhushan <address@hidden> wrote:
>
> 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?

ohh, We need to setup msi translation mapping, correct.

Thanks
-Bharat

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