virt: add irqfd trait and implement for mshv and KVM by will-j-wright · Pull Request #3206 · microsoft/openvmm

will-j-wright · 2026-04-06T21:17:22Z

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 · 2026-04-06T21:17:36Z

⚠️ Unsafe Code Detected

This PR modifies files containing unsafe Rust code. Extra scrutiny is required during review.

For more on why we check whole files, instead of just diffs, check out the Rustonomicon

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.

vmm_core/virt_mshv/src/irqfd.rs

vmm_core/virt_kvm/src/gsi.rs

Copilot

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 6 comments.

vmm_core/virt_mshv/src/irqfd.rs

vmm_core/virt_kvm/src/gsi.rs

vmm_core/virt_kvm/src/arch/x86_64/mod.rs

Copilot

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Copilot · 2026-04-07T21:14:19Z

vmm_core/virt_mshv/src/irqfd.rs

+        *self.last_route.lock() = Some(route);
+        self.shared.set_gsi_route(self.gsi, Some(route))
+    }
+
+    fn clear_msi(&self) -> anyhow::Result<()> {
+        *self.last_route.lock() = None;
+        self.shared.set_gsi_route(self.gsi, None)


last_route is updated before/without confirming the kernel routing update succeeded. If set_gsi_route fails (and rolls back the shared in-memory routing state), last_route can become inconsistent with the actual configured route, which can break unmask() semantics and error recovery. Update last_route only after set_gsi_route(...) succeeds (or roll it back on error); similarly, only clear last_route after successfully clearing the route.

Suggested change

*self.last_route.lock() = Some(route);

self.shared.set_gsi_route(self.gsi, Some(route))

}

fn clear_msi(&self) -> anyhow::Result<()> {

*self.last_route.lock() = None;

self.shared.set_gsi_route(self.gsi, None)

self.shared.set_gsi_route(self.gsi, Some(route))?;

*self.last_route.lock() = Some(route);

Ok(())

}

fn clear_msi(&self) -> anyhow::Result<()> {

self.shared.set_gsi_route(self.gsi, None)?;

*self.last_route.lock() = None;

Ok(())

Copilot · 2026-04-07T21:14:20Z

vmm_core/virt_mshv/src/irqfd.rs

+        // queried via is_pending() or consumed via consume_pending(). The
+        // last route is preserved so it can be restored on unmask.


The comment references is_pending(), but IrqFdRoute does not define such a method (only consume_pending). This is misleading for callers; either remove the is_pending() mention or align it with the actual API (e.g., only refer to consume_pending).

Suggested change

// queried via is_pending() or consumed via consume_pending(). The

// last route is preserved so it can be restored on unmask.

// consumed via consume_pending(). The last route is preserved so it

// can be restored on unmask.

Add irqfd support enabling the kernel to inject MSIs directly into the guest when an eventfd 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 in virt crate: - IrqFd: allocates GSIs and registers eventfds as irqfd routes - IrqFdRoute: updates/clears MSI routing (address/data) per GSI - Partition::irqfd(): returns the irqfd interface if supported mshv implementation (virt_mshv/src/irqfd.rs): - GSI allocation table (2048 slots) - MSHV_IRQFD ioctl for register/unregister eventfds - MSHV_SET_MSI_ROUTING ioctl to push full routing table - Automatic cleanup on route drop (unregister + free GSI) - VmFd changed to Arc<VmFd> for shared ownership KVM implementation (virt_kvm/src/gsi.rs): - KvmIrqFdState wraps existing GsiRoute infrastructure - KvmIrqFdRoute delegates to GsiRoute::enable/disable - Partition::irqfd() wired in x86_64 arch module Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

will-j-wright requested a review from a team as a code owner April 6, 2026 21:17

Copilot AI review requested due to automatic review settings April 6, 2026 21:17

github-actions bot added the unsafe Related to unsafe code label Apr 6, 2026

Copilot started reviewing on behalf of will-j-wright April 6, 2026 21:18 View session

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will-j-wright force-pushed the irqfd-pr branch from 45c1dcb to f3ed033 Compare April 6, 2026 22:19

jstarks reviewed Apr 7, 2026

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Copilot AI review requested due to automatic review settings April 7, 2026 17:55

will-j-wright force-pushed the irqfd-pr branch from f3ed033 to 931fcff Compare April 7, 2026 17:55

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Copilot started reviewing on behalf of will-j-wright April 7, 2026 18:27 View session

will-j-wright force-pushed the irqfd-pr branch from 931fcff to 8ea0732 Compare April 7, 2026 18:31

Copilot AI review requested due to automatic review settings April 7, 2026 21:06

will-j-wright force-pushed the irqfd-pr branch from 8ea0732 to 96ad88d Compare April 7, 2026 21:06

Copilot started reviewing on behalf of will-j-wright April 7, 2026 21:12 View session

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will-j-wright force-pushed the irqfd-pr branch from 96ad88d to ed2c758 Compare April 7, 2026 21:16

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		// queried via is_pending() or consumed via consume_pending(). The
		// last route is preserved so it can be restored on unmask.

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will-j-wright commented Apr 6, 2026

New traits

mshv implementation

KVM implementation

Testing

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