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

[Auger Eric]

Hi Bharat,

On 5/5/20 12:18 PM, Bharat Bhushan wrote:
> 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.
The virtio-iommu driver is supposed to obey the map() operation argument
and cascade this to the device. I don't see why it would filter this out.

Now we should have means to declare the vITS GPA as an HW MSI reserved
region and avoid calling the map(). I will try this out.

Thanks

Eric

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