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

[Peter Xu]

On Fri, Mar 31, 2017 at 12:21:23PM +0800, Jason Wang wrote:
> 
> 
> 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().

Do we need to?

VFIO will need it for building up shadow page table, even without a
vIOMMU. But imho that should not be needed by vhost, right?

-- peterx