Re: [PATCH 15/34] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory
From: Sean Christopherson <seanjc@google.com>
Date: 2025-10-07 13:58:20
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kvm, kvm-riscv, kvmarm, linux-arm-kernel, linux-fsdevel, linux-mips, linux-mm, linux-riscv, lkml
On Fri, Oct 03, 2025, Nikita Kalyazin wrote:
On 05/11/2023 16:30, Paolo Bonzini wrote:quoted
From: Sean Christopherson <seanjc@google.com> 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....quoted
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index f1a575d39b3b..8f46d757a2c5 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c...quoted
-static int check_memory_region_flags(const struct kvm_userspace_memory_region2 *mem) +static int check_memory_region_flags(struct kvm *kvm, + const struct kvm_userspace_memory_region2 *mem) { u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES; + if (kvm_arch_has_private_mem(kvm)) + valid_flags |= KVM_MEM_GUEST_MEMFD; + + /* Dirty logging private memory is not currently supported. */ + if (mem->flags & KVM_MEM_GUEST_MEMFD) + valid_flags &= ~KVM_MEM_LOG_DIRTY_PAGES;I was wondering whether this restriction is still required at this stage or can be lifted in cases where the guest memory is accessible by the host.
Off the top of my head, I can't think of any reason why dirty logging wouldn't work with guest_memfd for non-CoCo VMs. We'd likely need to explicitly enumerate support to userspace, and there might be some assumptions lurking in KVM, but fundamentally it should Just Work (TM).
Specifically, it would be useful to support differential memory snapshots based on dirty page tracking in Firecracker [1] or in live migration. As an experiment, I removed the check and was able to produce a diff snapshot and restore a Firecracker VM from it. [1] https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/snapshot-support.md#creating-diff-snapshotsquoted
+ #ifdef __KVM_HAVE_READONLY_MEM valid_flags |= KVM_MEM_READONLY; #endif@@ -2018,7 +2029,7 @@ int __kvm_set_memory_region(struct kvm *kvm, int as_id, id; int r; - r = check_memory_region_flags(mem); + r = check_memory_region_flags(kvm, mem); if (r) return r;