On 2023-10-27 11:21 AM, Sean Christopherson wrote: > 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). All of these use-cases involve using guest_memfd for shared pages, but this entire series sets up KVM to only use guest_memfd for private pages. For example, the per-page attributes are a property of a KVM VM, not the underlying guest_memfd. So that implies we will need separate guest_memfds for private and shared pages. But a given memslot can have a mix of private and shared pages. So that implies a memslot will need to support 2 guest_memfds? But the UAPI only allows 1 and uses the HVA for shared mappings. My initial reaction after reading through this series is that the per-page private/shared should be a property of the guest_memfd, not the VM. Maybe it would even be cleaner in the long-run to make all memory attributes a property of the guest_memfd. That way we can scope the support to only guest_memfds and not have to worry about making per-page attributes work with "legacy" HVA-based memslots. Maybe can you sketch out how you see this proposal being extensible to using guest_memfd for shared mappings?
