Hi David, > From: David Gibson [mailto:david@xxxxxxxxxxxxxxxxxxxxx] > Sent: Friday, January 31, 2020 11:59 AM > To: Liu, Yi L <yi.l.liu@xxxxxxxxx> > Subject: Re: [RFC v3 02/25] hw/iommu: introduce DualStageIOMMUObject > > On Wed, Jan 29, 2020 at 04:16:33AM -0800, Liu, Yi L wrote: > > From: Liu Yi L <yi.l.liu@xxxxxxxxx> > > > > Currently, many platform vendors provide the capability of dual stage > > DMA address translation in hardware. For example, nested translation > > on Intel VT-d scalable mode, nested stage translation on ARM SMMUv3, > > and etc. In dual stage DMA address translation, there are two stages > > address translation, stage-1 (a.k.a first-level) and stage-2 (a.k.a > > second-level) translation structures. Stage-1 translation results are > > also subjected to stage-2 translation structures. Take vSVA (Virtual > > Shared Virtual Addressing) as an example, guest IOMMU driver owns > > stage-1 translation structures (covers GVA->GPA translation), and host > > IOMMU driver owns stage-2 translation structures (covers GPA->HPA > > translation). VMM is responsible to bind stage-1 translation structures > > to host, thus hardware could achieve GVA->GPA and then GPA->HPA > > translation. For more background on SVA, refer the below links. > > - https://www.youtube.com/watch?v=Kq_nfGK5MwQ > > - https://events19.lfasiallc.com/wp-content/uploads/2017/11/\ > > Shared-Virtual-Memory-in-KVM_Yi-Liu.pdf > > > > As above, dual stage DMA translation offers two stage address mappings, > > which could have better DMA address translation support for passthru > > devices. This is also what vIOMMU developers are doing so far. Efforts > > includes vSVA enabling from Yi Liu and SMMUv3 Nested Stage Setup from > > Eric Auger. > > https://www.spinics.net/lists/kvm/msg198556.html > > https://lists.gnu.org/archive/html/qemu-devel/2019-07/msg02842.html > > > > Both efforts are aiming to expose a vIOMMU with dual stage hardware > > backed. As so, QEMU needs to have an explicit object to stand for > > the dual stage capability from hardware. Such object offers abstract > > for the dual stage DMA translation related operations, like: > > > > 1) PASID allocation (allow host to intercept in PASID allocation) > > 2) bind stage-1 translation structures to host > > 3) propagate stage-1 cache invalidation to host > > 4) DMA address translation fault (I/O page fault) servicing etc. > > > > This patch introduces DualStageIOMMUObject to stand for the hardware > > dual stage DMA translation capability. PASID allocation/free are the > > first operation included in it, in future, there will be more operations > > like bind_stage1_pgtbl and invalidate_stage1_cache and etc. > > > > Cc: Kevin Tian <kevin.tian@xxxxxxxxx> > > Cc: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx> > > Cc: Peter Xu <peterx@xxxxxxxxxx> > > Cc: Eric Auger <eric.auger@xxxxxxxxxx> > > Cc: Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx> > > Cc: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx> > > Several overall queries about this: > > 1) Since it's explicitly handling PASIDs, this seems a lot more > specific to SVM than the name suggests. I'd suggest a rename. It is not specific to SVM in future. We have efforts to move guest IOVA support based on host IOMMU's dual-stage DMA translation capability. Then, guest IOVA support will also re-use the methods provided by this abstract layer. e.g. the bind_guest_pgtbl() and flush_iommu_iotlb(). For the naming, how about HostIOMMUContext? This layer is to provide explicit methods for setting up dual-stage DMA translation in host. > > 2) Why are you hand rolling structures of pointers, rather than making > this a QOM class or interface and putting those things into methods? Maybe the name is not proper. Although I named it as DualStageIOMMUObject, it is actually a kind of abstract layer we discussed in previous email. I think this is similar with VFIO_MAP/UNMAP. The difference is that VFIO_MAP/ UNMAP programs mappings to host iommu domain. While the newly added explicit method is to link guest page table to host iommu domain. VFIO_MAP/UNMAP is exposed to vIOMMU emulators via MemoryRegion layer. right? Maybe adding a similar abstract layer is enough. Is adding QOM really necessary for this case? > 3) It's not really clear to me if this is for the case where both > stages of translation are visible to the guest, or only one of > them. For this case, vIOMMU will only expose a single stage translation to VM. e.g. Intel VT-d, vIOMMU exposes first-level translation to guest. Hardware IOMMUs with the dual-stage translation capability lets guest own stage-1 translation structures and host owns the stage-2 translation structures. VMM is responsible to bind guest's translation structures to host and enable dual-stage translation. e.g. on Intel VT-d, config translation type to be NESTED. Take guest SVM as an example, guest iommu driver owns the gVA->gPA mappings, which is treated as stage-1 translation from host point of view. Host itself owns the gPA->hPPA translation and called stage-2 translation when dual-stage translation is configured. For guest IOVA, it is similar with guest SVM. Guest iommu driver owns the gIOVA->gPA mappings, which is treated as stage-1 translation. Host owns the gPA->hPA translation. Regards, Yi Liu
