This is the v5 series of the shared device assignment support. As discussed in the v4 series [1], the GenericStateManager parent class and PrivateSharedManager child interface were deemed to be in the wrong direction. This series reverts back to the original single RamDiscardManager interface and puts it as future work to allow the co-existence of multiple pairs of state management. For example, if we want to have virtio-mem co-exist with guest_memfd, it will need a new framework to combine the private/shared/discard states [2].
Another change since the last version is the error handling of memory conversion. Currently, the failure of kvm_convert_memory() causes QEMU to quit instead of resuming the guest. The complex rollback operation doesn't add value and merely adds code that is difficult to test. Although in the future, it is more likely to encounter more errors on conversion paths like unmap failure on shared to private in-place conversion. This series keeps complex error handling out of the picture for now and attaches related handling at the end of the series for future extension. Apart from the above two parts with future work, there's some optimization work in the future, i.e., using other more memory-efficient mechanism to track ranges of contiguous states instead of a bitmap [3]. This series still uses a bitmap for simplicity. The overview of this series: - Patch 1-3: Preparation patches. These include function exposure and some definition changes to return values. - Patch 4-5: Introduce a new object to implement RamDiscardManager interface and a helper to notify the shared/private state change. - Patch 6: Store the new object including guest_memfd information in RAMBlock. Register the RamDiscardManager instance to the target RAMBlock's MemoryRegion so that the RamDiscardManager users can run in the specific path. - Patch 7: Unlock the coordinate discard so that the shared device assignment (VFIO) can work with guest_memfd. After this patch, the basic device assignement functionality can work properly. - Patch 8-9: Some cleanup work. Move the state change handling into a RamDiscardListener so that it can be invoked together with the VFIO listener by the state_change() call. This series dropped the priority support in v4 which is required by in-place conversions, because the conversion path will likely change. - Patch 10: More complex error handing including rollback and mixture states conversion case. More small changes or details can be found in the individual patches. --- Original cover letter: Background ========== Confidential VMs have two classes of memory: shared and private memory. Shared memory is accessible from the host/VMM while private memory is not. Confidential VMs can decide which memory is shared/private and convert memory between shared/private at runtime. "guest_memfd" is a new kind of fd whose primary goal is to serve guest private memory. In current implementation, shared memory is allocated with normal methods (e.g. mmap or fallocate) while private memory is allocated from guest_memfd. When a VM performs memory conversions, QEMU frees pages via madvise or via PUNCH_HOLE on memfd or guest_memfd from one side, and allocates new pages from the other side. This will cause a stale IOMMU mapping issue mentioned in [4] when we try to enable shared device assignment in confidential VMs. Solution ======== The key to enable shared device assignment is to update the IOMMU mappings on page conversion. RamDiscardManager, an existing interface currently utilized by virtio-mem, offers a means to modify IOMMU mappings in accordance with VM page assignment. Although the required operations in VFIO for page conversion are similar to memory plug/unplug, the states of private/shared are different from discard/populated. We want a similar mechanism with RamDiscardManager but used to manage the state of private and shared. This series introduce a new parent abstract class to manage a pair of opposite states with RamDiscardManager as its child to manage populate/discard states, and introduce a new child class, PrivateSharedManager, which can also utilize the same infrastructure to notify VFIO of page conversions. Relationship with in-place page conversion ========================================== To support 1G page support for guest_memfd [5], the current direction is to allow mmap() of guest_memfd to userspace so that both private and shared memory can use the same physical pages as the backend. This in-place page conversion design eliminates the need to discard pages during shared/private conversions. However, device assignment will still be blocked because the in-place page conversion will reject the conversion when the page is pinned by VFIO. To address this, the key difference lies in the sequence of VFIO map/unmap operations and the page conversion. It can be adjusted to achieve unmap-before-conversion-to-private and map-after-conversion-to-shared, ensuring compatibility with guest_memfd. Limitation ========== One limitation is that VFIO expects the DMA mapping for a specific IOVA to be mapped and unmapped with the same granularity. The guest may perform partial conversions, such as converting a small region within a larger region. To prevent such invalid cases, all operations are performed with 4K granularity. This could be optimized after the cut_mapping operation[6] is introduced in future. We can alway perform a split-before-unmap if partial conversions happen. If the split succeeds, the unmap will succeed and be atomic. If the split fails, the unmap process fails. Testing ======= This patch series is tested based on TDX patches available at: KVM: https://github.com/intel/tdx/tree/kvm-coco-queue-snapshot/kvm-coco-queue-snapshot-20250408 QEMU: https://github.com/intel-staging/qemu-tdx/tree/tdx-upstream-snapshot-2025-05-20 Because the new features like cut_mapping operation will only be support in iommufd. It is recommended to use the iommufd-backed VFIO with the qemu command: qemu-system-x86_64 [...] -object iommufd,id=iommufd0 \ -device vfio-pci,host=XX:XX.X,iommufd=iommufd0 Following the bootup of the TD guest, the guest's IP address becomes visible, and iperf is able to successfully send and receive data. Related link ============ [1] https://lore.kernel.org/qemu-devel/20250407074939.18657-1-chenyi.qi...@intel.com/ [2] https://lore.kernel.org/qemu-devel/d1a71e00-243b-4751-ab73-c05a4e090...@redhat.com/ [3] https://lore.kernel.org/qemu-devel/96ab7fa9-bd7a-444d-aef8-8c9c30439...@redhat.com/ [4] https://lore.kernel.org/qemu-devel/20240423150951.41600-54-pbonz...@redhat.com/ [5] https://lore.kernel.org/kvm/cover.1747264138.git.ackerley...@google.com/ [6] https://lore.kernel.org/linux-iommu/0-v2-5c26bde5c22d+58b-iommu_pt_...@nvidia.com/ Chenyi Qiang (10): memory: Export a helper to get intersection of a MemoryRegionSection with a given range memory: Change memory_region_set_ram_discard_manager() to return the result memory: Unify the definiton of ReplayRamPopulate() and ReplayRamDiscard() ram-block-attribute: Introduce RamBlockAttribute to manage RAMBlock with guest_memfd ram-block-attribute: Introduce a helper to notify shared/private state changes memory: Attach RamBlockAttribute to guest_memfd-backed RAMBlocks RAMBlock: Make guest_memfd require coordinate discard memory: Change NotifyRamDiscard() definition to return the result KVM: Introduce RamDiscardListener for attribute changes during memory conversions ram-block-attribute: Add more error handling during state changes MAINTAINERS | 1 + accel/kvm/kvm-all.c | 79 ++- hw/vfio/listener.c | 6 +- hw/virtio/virtio-mem.c | 83 ++-- include/system/confidential-guest-support.h | 9 + include/system/memory.h | 76 ++- include/system/ramblock.h | 22 + migration/ram.c | 33 +- system/memory.c | 22 +- system/meson.build | 1 + system/physmem.c | 18 +- system/ram-block-attribute.c | 514 ++++++++++++++++++++ target/i386/kvm/tdx.c | 1 + target/i386/sev.c | 1 + 14 files changed, 770 insertions(+), 96 deletions(-) create mode 100644 system/ram-block-attribute.c -- 2.43.5
