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

[Jason Wang]

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