windows: Fix polly and utils crates to compile on Windows#660
Merged
slp merged 4 commits intocontainers:mainfrom May 5, 2026
Merged
windows: Fix polly and utils crates to compile on Windows#660slp merged 4 commits intocontainers:mainfrom
slp merged 4 commits intocontainers:mainfrom
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lstocchi
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MatiasVara,
dorindabassey,
jakecorrenti,
mtjhrc,
slp and
tylerfanelli
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mtjhrc
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May 4, 2026
This introduces an epoll-compatible polling abstraction for Windows by leveraging I/O Completion Ports (IOCP) as a central event multiplexer. By bypassing the standard `WaitForMultipleObjects` 64-handle limit, this architecture gives us true O(1) wake-ups with no handle-count limitations. When an event is added to the epoll, we heap-allocate a `Watch` struct and attach a Wait Completion Packet (WCP) to it. The raw memory pointer of this struct is passed to the kernel as the completion key. When the event signals, the kernel pushes the WCP to the IOCP queue. The waiting thread pops the packet and reads the pointer, allowing us to process events with zero heap allocations and a completely lock-free hot path. When deleting an event, the WCP is closed and the `Watch` memory is moved to a "zombie" list with a 5-second garbage collection delay. Because the IOCP queue is managed asynchronously by the Windows kernel, a deleted event might still have a completion packet in-flight to a worker thread. This GC window safely drains those ghost packets before freeing the memory, preventing Use-After-Free segfaults. One calculated tradeoff is the reliance on the undocumented Windows NT native API (`NtAssociateWaitCompletionPacket`). While unofficial, it has been stable in the Windows kernel for over a decade and is currently the only way to achieve VMM-grade, O(1) polling performance for arbitrary handles. Additional details: - EventSet bit values mirror the macOS implementation for cross-platform portability. - Adds `windows-sys` as a Windows-only dependency for OS APIs. Signed-off-by: lstocchi <lstocchi@redhat.com>
Emulate eventfd on Windows using a manual-reset kernel Event object paired with a Mutex-protected counter. To maximize VMM throughput, the `write` path only trigger the event when the counter transitions from `0 -> non-zero`. If a virtual device rapid-fires multiple interrupts before the vCPU wakes up, we accumulate the data in user-space RAM and skip the redundant kernel syscalls entirely. `read` and `wait_timeout` maintain strict level-triggered synchronization. The kernel event is only reset (`ResetEvent`) when the internal counter is fully drained, preventing the IOCP epoll loop from entering an infinite busy-wait cycle. Signed-off-by: lstocchi <lstocchi@redhat.com>
cfg-gate dependencies and implementations based on the target OS so that the utils crate also compile on Windows Signed-off-by: lstocchi <lstocchi@redhat.com>
Make the polly event manager compile and pass tests on Windows by replacing Unix-specific types and APIs with platform-gated equivalents Signed-off-by: lstocchi <lstocchi@redhat.com>
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LGTM, thanks @lstocchi. Do you think the unit tests for Windows could be run in a GitHub runner (at least, the ones not requiring WHPX). |
slp
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May 5, 2026
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@slp yes, we should definitely add it. The problem is that we cannot use cargo test right now bc of the compilation issue with vmm-rust deps. Maybe we could target the specific crates that are windows compatible? E.g. cargo test -p krun-polly .... |
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This is the second PR for porting libkrun on Windows. Nothing fancy and relatively small.
I'm just going back through the code I wrote for my tests over the last few weeks. Cleaning it up and pushing it with a bit more organization. Next would be the WHP crate.
As @slp confirmed I'm going to push to main.
As you can see the first two commits are the same as #609 (edit 5/5/26: updated epoll to support the change made in #612) because the main do not contain them. If you prefer we can merge them separately and rebase this PR.
Overall, it's just a bunch of instructions to gate unix-related functions which won't be used on Windows and replace the misssing one with Windows code.