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The userspace unmaps a DMA >>>>> page and maps it back right away, all from the userspace? The end >>>>> result will be the exactly same which seems useless. And IOMMU TLB >>> >>> As Jason described, ideally IOMMU just like KVM, should just: >>> 1) Directly rely on guest_memfd for pinning -> no page refcounts taken >>> by IOMMU stack >>> 2) Directly query pfns from guest_memfd for both shared/private ranges >>> 3) Implement an invalidation callback that guest_memfd can invoke on >>> conversions. > > Conversions and truncations both. > >>> >>> Current flow: >>> Private to Shared conversion via kvm_gmem_convert_range() - >>> 1) guest_memfd invokes kvm_gmem_invalidate_begin() for the ranges >>> on each bound memslot overlapping with the range >>> -> KVM has the concept of invalidation_begin() and end(), >>> which effectively ensures that between these function calls, no new >>> EPT/NPT entries can be added for the range. >>> 2) guest_memfd invokes kvm_gmem_convert_should_proceed() which >>> actually unmaps the KVM SEPT/NPT entries. >>> 3) guest_memfd invokes kvm_gmem_execute_work() which updates the >>> shareability and then splits the folios if needed >>> >>> Shared to private conversion via kvm_gmem_convert_range() - >>> 1) guest_memfd invokes kvm_gmem_invalidate_begin() for the ranges >>> on each bound memslot overlapping with the range >>> 2) guest_memfd invokes kvm_gmem_convert_should_proceed() which >>> actually unmaps the host mappings which will unmap the KVM non-seucure >>> EPT/NPT entries. >>> 3) guest_memfd invokes kvm_gmem_execute_work() which updates the >>> shareability and then merges the folios if needed. >>> >>> ============================ >>> >>> For IOMMU, could something like below work? >>> >>> * A new UAPI to bind IOMMU FDs with guest_memfd ranges >> >> Done that. >> >>> * VFIO_DMA_MAP/UNMAP operations modified to directly fetch pfns from >>> guest_memfd ranges using kvm_gmem_get_pfn() >> >> This API imho should drop the confusing kvm_ prefix. >> >>> -> kvm invokes kvm_gmem_is_private() to check for the range >>> shareability, IOMMU could use the same or we could add an API in gmem >>> that takes in access type and checks the shareability before returning >>> the pfn. >> >> Right now I cutnpasted kvm_gmem_get_folio() (which essentially is filemap_lock_folio()/filemap_alloc_folio()/__filemap_add_folio()) to avoid new links between iommufd.ko and kvm.ko. It is probably unavoidable though. > > I don't think that's the way to avoid links between iommufd.ko and > kvm.ko. Cleaner way probably is to have gmem logic built-in and allow > runtime registration of invalidation callbacks from KVM/IOMMU > backends. Need to think about this more. Yeah, otherwise iommufd.ko will have to install a hook in guest_memfd (==kvm.ko) in run time so more beloved symbol_get() :) > >> >> >>> * IOMMU stack exposes an invalidation callback that can be invoked by >>> guest_memfd. >>> >>> Private to Shared conversion via kvm_gmem_convert_range() - >>> 1) guest_memfd invokes kvm_gmem_invalidate_begin() for the ranges >>> on each bound memslot overlapping with the range >>> 2) guest_memfd invokes kvm_gmem_convert_should_proceed() which >>> actually unmaps the KVM SEPT/NPT entries. >>> -> guest_memfd invokes IOMMU invalidation callback to zap >>> the secure IOMMU entries. >>> 3) guest_memfd invokes kvm_gmem_execute_work() which updates the >>> shareability and then splits the folios if needed >>> 4) Userspace invokes IOMMU map operation to map the ranges in >>> non-secure IOMMU. >>> >>> Shared to private conversion via kvm_gmem_convert_range() - >>> 1) guest_memfd invokes kvm_gmem_invalidate_begin() for the ranges >>> on each bound memslot overlapping with the range >>> 2) guest_memfd invokes kvm_gmem_convert_should_proceed() which >>> actually unmaps the host mappings which will unmap the KVM non-seucure >>> EPT/NPT entries. >>> -> guest_memfd invokes IOMMU invalidation callback to zap the >>> non-secure IOMMU entries. >>> 3) guest_memfd invokes kvm_gmem_execute_work() which updates the >>> shareability and then merges the folios if needed. >>> 4) Userspace invokes IOMMU map operation to map the ranges in secure IOMMU. >> >> >> Alright (although this zap+map is not necessary on the AMD hw). > > IMO guest_memfd ideally should not directly interact or cater to arch > specific needs, it should implement a mechanism that works for all > archs. KVM/IOMMU implement invalidation callbacks and have all the > architecture specific knowledge to take the right decisions. Every page conversion will go through: kvm-amd.ko -1-> guest_memfd (kvm.ko) -2-> iommufd.ko -3-> amd-iommu (build-in). Which one decides on IOMMU not needing (un)mapping? Got to be (1) but then it need to propagate the decision to amd-iommu (and we do not have (3) at the moment in that path). Or we just always do unmap+map (and trigger unwanted page huge page smashing)? All is doable and neither particularly horrible, I'm trying to see where the consensus is now. Thanks, >> >>> There should be a way to block external IOMMU pagetable updates while >>> guest_memfd is performing conversion e.g. something like >>> kvm_invalidate_begin()/end(). >>> >>>>> is going to be flushed on a page conversion anyway (the RMPUPDATE >>>>> instruction does that). All this is about AMD's x86 though. >>>> >>>> The iommu should not be using the VMA to manage the mapping. It should >>> >>> +1. >> >> Yeah, not doing this already, because I physically cannot map gmemfd's memory in IOMMU via VMA (which allocates memory via gup() so wrong memory is mapped in IOMMU). Thanks, >> >> >>>> be directly linked to the guestmemfd in some way that does not disturb >>>> its operations. I imagine there would be some kind of invalidation >>>> callback directly to the iommu. >>>> >>>> Presumably that invalidation call back can include a reason for the >>>> invalidation (addr change, shared/private conversion, etc) >>>> >>>> I'm not sure how we will figure out which case is which but guestmemfd >>>> should allow the iommu to plug in either invalidation scheme.. >>>> >>>> Probably invalidation should be a global to the FD thing, I imagine >>>> that once invalidation is established the iommu will not be >>>> incrementing page refcounts. >>> >>> +1. >> >> Alright. Thanks for the comments. >> >>> >>>> >>>> Jason >> >> -- >> Alexey >> -- Alexey