Re: [PATCH v4 05/22] iommu: introduce iommu invalidate API function

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

On Wed, 2 May 2018 10:31:50 +0100
Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:

> On 01/05/18 23:58, Jacob Pan wrote:
> >>>> Maybe this should be called "NG_PAGE_PASID",    
> >>> Sure. I was thinking page range already implies non-global
> >>> pages.    
> >>>> and "DOMAIN_PAGE" should
> >>>> instead be "PAGE_PASID". If I understood their meaning correctly,
> >>>> it would be more consistent with the rest.
> >>>>    
> >>> I am trying not to mix granu between request w/ PASID and w/o.
> >>> DOMAIN_PAGE meant to be for request w/o PASID.    
> >>
> >> Is the distinction necessary? I understand the IOMMU side might
> >> offer many possibilities for invalidation, but the user probably
> >> doesn't need all of them. It might be easier to document, upstream
> >> and maintain if we only specify what's currently needed by users
> >> (what does QEMU VT-d use?) Others can always extend it by
> >> increasing the version.
> >>
> >> Do you think that this invalidation message will be used outside of
> >> BIND_PASID_TABLE context? I can't see an other use but who knows.
> >> At the moment requests w/o PASID are managed with
> >> VFIO_IOMMU_MAP/UNMAP_DMA, which doesn't require invalidation. And
> >> in a BIND_PASID_TABLE context, IOMMUs requests w/o PASID are just a
> >> special case using PASID 0 (for Arm and AMD) so I suppose they'll
> >> use the same invalidation commands as requests w/ PASID.
> >>  
> > My understanding is that for GIOVA use case, VT-d vIOMMU creates
> > GIOVA-GPA mapping and the host shadows the 2nd level page tables to
> > create GIOVA-HPA mapping. So when assigned device in the guest can
> > do both DMA map/unmap and VFIO map/unmap, VFIO unmap is one time
> > deal (I guess invalidation can be captured in other code path), but
> > guest kernel use of DMA unmap could will trigger invalidation. QEMU
> > needs to trap those invalidation and passdown to physical IOMMU. So
> > we do need invalidation w/o PASID.  
> 
> Hm, isn't this all done by host userspace? Whether guest does DMA
> map/unmap or VFIO map/unmap, it creates/removes IOVA-GPA mappings in
> the vIOMMU. QEMU captures invalidation requests for these mappings
> from the guest, finds GPA-HVA in the shadow map and sends a VFIO
> map/unmap request for IOVA-HVA.
> 
Sorry for the delay but you are right, I have also confirmed with Yi
that we don't need second level invalidation. I will remove IOTLB
invalidation w/o PASID case from the API.

Thanks,

> Thanks,
> Jean
> 

[Jacob Pan]