On Tue, Jul 14, 2015 at 10:21:54PM +1000, Alexey Kardashevskiy wrote: > This makes use of the new "memory registering" feature. The idea is > to provide the userspace ability to notify the host kernel about pages > which are going to be used for DMA. Having this information, the host > kernel can pin them all once per user process, do locked pages > accounting (once) and not spent time on doing that in real time with > possible failures which cannot be handled nicely in some cases. > > This adds a guest RAM memory listener which notifies a VFIO container > about memory which needs to be pinned/unpinned. VFIO MMIO regions > (i.e. "skip dump" regions) are skipped. > > The feature is only enabled for SPAPR IOMMU v2. The host kernel changes > are required. Since v2 does not need/support VFIO_IOMMU_ENABLE, this does > not call it when v2 is detected and enabled. > > This does not change the guest visible interface. > > Signed-off-by: Alexey Kardashevskiy <a...@ozlabs.ru>
I've looked at this in more depth now, and attempting to unify the pre-reg and mapping listeners like this can't work - they need to be listening on different address spaces: mapping actions need to be listening on the PCI address space, whereas the pre-reg needs to be listening on address_space_memory. For x86 - for now - those end up being the same thing, but on Power they're not. We do need to be clear about what differences are due to the presence of a guest IOMMU versus which are due to arch or underlying IOMMU type. For now Power has a guest IOMMU and x86 doesn't, but that could well change in future: we could well implement the guest side IOMMU for x86 in future (or x86 could invent a paravirt IOMMU interface). On the other side, BenH's experimental powernv machine type could introduce Power machines without a guest side IOMMU (or at least an optional guest side IOMMU). The quick and dirty approach here is: 1. Leave the main listener as is 2. Add a new pre-reg notifier to the spapr iommu specific code, which listens on address_space_memory, *not* the PCI space The more generally correct approach, which allows for more complex IOMMU arrangements and the possibility of new IOMMU types with pre-reg is: 1. Have the core implement both a mapping listener and a pre-reg listener (optionally enabled by a per-iommu-type flag). Basically the first one sees what *is* mapped, the second sees what *could* be mapped. 2. As now, the mapping listener listens on PCI address space, if RAM blocks are added, immediately map them into the host IOMMU, if guest IOMMU blocks appear register a notifier which will mirror guest IOMMU mappings to the host IOMMU (this is what we do now). 3. The pre-reg listener also listens on the PCI address space. RAM blocks added are pre-registered immediately. But, if guest IOMMU blocks are added, instead of registering a guest-iommu notifier, we register another listener on the *target* AS of the guest IOMMU, same callbacks as this one. In practice that target AS will almost always resolve to address_space_memory, but this can at least in theory handle crazy guest setups with multiple layers of IOMMU. 4. Have to ensure that the pre-reg callbacks always happen before the mapping calls. For a system with an IOMMU backend which requires pre-registration, but doesn't have a guest IOMMU, we need to pre-reg, then host-iommu-map RAM blocks that appear in PCI address space. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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