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

[Auger Eric]

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.

Thanks

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