virt: add irqfd trait and implement for mshv and KVM

[will-j-wright]

Add irqfd support enabling the kernel to inject MSIs directly into the guest when an event is signaled, without a userspace transition. This is required for VFIO device passthrough where physical device interrupts must be delivered to the guest via the hypervisor's irqfd mechanism.

New traits

• IrqFd — creates irqfd routes (GSI allocation + event registration)
• IrqFdRoute — manages per-vector MSI routing, masking, and pending state for PBA support
• Partition::irqfd() — returns the irqfd interface if the backend supports it

The event is created by the implementation rather than provided by the caller. This accommodates backends like WHP where the hypervisor owns the event handle (WHvCreateTrigger).

mshv implementation

• GSI allocation table (2048 slots)
• MSHV_IRQFD ioctl for register/unregister eventfds
• MSHV_SET_MSI_ROUTING ioctl to push full routing table atomically
• Automatic cleanup on route drop; GSI only freed on successful unregister

KVM implementation

• KvmIrqFdState wraps existing GsiRoute infrastructure
• KvmIrqFdRoute delegates to GsiRoute::enable/disable

Testing

End-to-end with VFIO NVMe passthrough on mshv.

[github-actions]

⚠️ Unsafe Code Detected

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[Copilot]

Pull request overview

This PR introduces an irqfd abstraction to the virt layer and wires up backend implementations for mshv and KVM, enabling kernel-assisted MSI injection (needed for VFIO-style interrupt delivery without userspace transitions).

Changes:

• Add new virt::irqfd module defining IrqFd / IrqFdRoute traits and Partition::irqfd() capability hook.
• Implement irqfd + MSI routing for mshv (GSI allocation, MSHV_IRQFD, atomic MSHV_SET_MSI_ROUTING updates).
• Implement irqfd support for KVM by wrapping existing GsiRoute infrastructure and exposing it via Partition::irqfd() on x86_64.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
vmm_core/virt/src/lib.rs	Exposes the new irqfd module.
vmm_core/virt/src/irqfd.rs	Adds IrqFd / IrqFdRoute traits and semantics for MSI routing + mask/unmask behavior.
vmm_core/virt/src/generic.rs	Adds Partition::irqfd() optional capability entrypoint.
vmm_core/virt_mshv/src/lib.rs	Stores shared MshvIrqFdState in the partition and exposes it via Partition::irqfd().
vmm_core/virt_mshv/src/irqfd.rs	New mshv irqfd implementation: GSI table + irqfd registration + full routing table push.
vmm_core/virt_mshv/Cargo.toml	Adds headervec dependency for variable-length ioctl buffer building.
vmm_core/virt_kvm/src/gsi.rs	Adds KvmIrqFdState/KvmIrqFdRoute wrapper implementing the new traits.
vmm_core/virt_kvm/src/arch/x86_64/mod.rs	Wires Partition::irqfd() for KVM x86_64 partitions.