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KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing me…
…mory Introduce an ioctl(), KVM_CREATE_GUEST_MEMFD, to allow creating file-based memory that is tied to a specific KVM virtual machine and whose primary purpose is to serve guest memory. A guest-first memory subsystem allows for optimizations and enhancements that are kludgy or outright infeasible to implement/support in a generic memory subsystem. With guest_memfd, guest protections and mapping sizes are fully decoupled from host userspace mappings. E.g. KVM currently doesn't support mapping memory as writable in the guest without it also being writable in host userspace, as KVM's ABI uses VMA protections to define the allow guest protection. Userspace can fudge this by establishing two mappings, a writable mapping for the guest and readable one for itself, but that’s suboptimal on multiple fronts. Similarly, KVM currently requires the guest mapping size to be a strict subset of the host userspace mapping size, e.g. KVM doesn’t support creating a 1GiB guest mapping unless userspace also has a 1GiB guest mapping. Decoupling the mappings sizes would allow userspace to precisely map only what is needed without impacting guest performance, e.g. to harden against unintentional accesses to guest memory. Decoupling guest and userspace mappings may also allow for a cleaner alternative to high-granularity mappings for HugeTLB, which has reached a bit of an impasse and is unlikely to ever be merged. A guest-first memory subsystem also provides clearer line of sight to things like a dedicated memory pool (for slice-of-hardware VMs) and elimination of "struct page" (for offload setups where userspace _never_ needs to mmap() guest memory). More immediately, being able to map memory into KVM guests without mapping said memory into the host is critical for Confidential VMs (CoCo VMs), the initial use case for guest_memfd. While AMD's SEV and Intel's TDX prevent untrusted software from reading guest private data by encrypting guest memory with a key that isn't usable by the untrusted host, projects such as Protected KVM (pKVM) provide confidentiality and integrity *without* relying on memory encryption. And with SEV-SNP and TDX, accessing guest private memory can be fatal to the host, i.e. KVM must be prevent host userspace from accessing guest memory irrespective of hardware behavior. Attempt #1 to support CoCo VMs was to add a VMA flag to mark memory as being mappable only by KVM (or a similarly enlightened kernel subsystem). That approach was abandoned largely due to it needing to play games with PROT_NONE to prevent userspace from accessing guest memory. Attempt #2 to was to usurp PG_hwpoison to prevent the host from mapping guest private memory into userspace, but that approach failed to meet several requirements for software-based CoCo VMs, e.g. pKVM, as the kernel wouldn't easily be able to enforce a 1:1 page:guest association, let alone a 1:1 pfn:gfn mapping. And using PG_hwpoison does not work for memory that isn't backed by 'struct page', e.g. if devices gain support for exposing encrypted memory regions to guests. Attempt #3 was to extend the memfd() syscall and wrap shmem to provide dedicated file-based guest memory. That approach made it as far as v10 before feedback from Hugh Dickins and Christian Brauner (and others) led to it demise. Hugh's objection was that piggybacking shmem made no sense for KVM's use case as KVM didn't actually *want* the features provided by shmem. I.e. KVM was using memfd() and shmem to avoid having to manage memory directly, not because memfd() and shmem were the optimal solution, e.g. things like read/write/mmap in shmem were dead weight. Christian pointed out flaws with implementing a partial overlay (wrapping only _some_ of shmem), e.g. poking at inode_operations or super_operations would show shmem stuff, but address_space_operations and file_operations would show KVM's overlay. Paraphrashing heavily, Christian suggested KVM stop being lazy and create a proper API. Link: https://lore.kernel.org/all/[email protected] Link: https://lore.kernel.org/all/[email protected] Link: https://lore.kernel.org/all/[email protected] Link: https://lore.kernel.org/all/[email protected] Link: https://lore.kernel.org/all/[email protected] Link: https://lore.kernel.org/all/[email protected] Link: https://lore.kernel.org/all/20230418-anfallen-irdisch-6993a61be10b@brauner Link: https://lore.kernel.org/all/[email protected] Link: https://lore.kernel.org/linux-mm/20230306191944.GA15773@monkey Link: https://lore.kernel.org/linux-mm/[email protected] Cc: Fuad Tabba <[email protected]> Cc: Vishal Annapurve <[email protected]> Cc: Ackerley Tng <[email protected]> Cc: Jarkko Sakkinen <[email protected]> Cc: Maciej Szmigiero <[email protected]> Cc: Vlastimil Babka <[email protected]> Cc: David Hildenbrand <[email protected]> Cc: Quentin Perret <[email protected]> Cc: Michael Roth <[email protected]> Cc: Wang <[email protected]> Cc: Liam Merwick <[email protected]> Cc: Isaku Yamahata <[email protected]> Co-developed-by: Kirill A. Shutemov <[email protected]> Signed-off-by: Kirill A. Shutemov <[email protected]> Co-developed-by: Yu Zhang <[email protected]> Signed-off-by: Yu Zhang <[email protected]> Co-developed-by: Chao Peng <[email protected]> Signed-off-by: Chao Peng <[email protected]> Co-developed-by: Ackerley Tng <[email protected]> Signed-off-by: Ackerley Tng <[email protected]> Co-developed-by: Isaku Yamahata <[email protected]> Signed-off-by: Isaku Yamahata <[email protected]> Co-developed-by: Paolo Bonzini <[email protected]> Signed-off-by: Paolo Bonzini <[email protected]> Co-developed-by: Michael Roth <[email protected]> Signed-off-by: Michael Roth <[email protected]> Signed-off-by: Sean Christopherson <[email protected]> Link: https://lore.kernel.org/r/[email protected]
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