Re: [Qemu-devel] [PATCH v7 14/17] memory: add MemoryRegionIOMMUOps.replay() callback

[Jason Wang <jasowang@redhat.com>]

On 2017年03月31日 10:56, Peter Xu wrote:
>>> Just come to mind that there may be a corner case here.
>>>
>>> Intel VT-d actually has a "pt" mode which allows device use physical address
>>> even when VT-d is enabled. In kernel, there is a iommu_identity_mapping.
>>> If a device is in this map, then it would use "pt" mode. So that IOMMU driver
>>> would not build second-level page table for it.
>> Yes, but qemu does not support ECAP_PT now, so guest will still have a page
>> table in this case.
>>
>>> Back to the virtual IOVA implementation, if an assigned device is in the
>>> iommu_identity_mapping(e.g. VGA controller), it uses GPA directly to do DMA.
>>> So it demands a GPA->HPA mapping in host. However, the iommu->ops.replay
>>> is not able to build it when guest SL page table is empty.
>>>
>>> So I think building an entire guest PA->HPA mapping before guest kernel boot
>>> would be recommended. Any thoughts?
>> We plan to add PT in 2.10, a possible rough idea is disabled iommu dmar
>> region and use another region without iommu_ops. Then
>> vfio_listener_region_add() will just do the correct mappings.
> Even without any new region. With the patch 16/17 ("intel_iommu: allow
> dynamic switch of IOMMU region"), we can just turn the IOMMU region
> on/off, following the device's PT bit, maybe using the new
> vtd_switch_address_space() interface. That should be enough.

Right. For vhost it was probably need more works, e.g setting up static 
mappings during region_add().

>
> Again, we just need to wait until current series merged.
>
> (Oh, then I found why I had an extra "on/off" parameter in previous
>   versions in vtd_switch_address_space(), but it was removed.)

Good to know this.

Thanks

>
> Thanks,
>
> -- peterx