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boot: 0.931
device: 0.926
instruction: 0.881
vnc: 0.865
other: 0.853
semantic: 0.851
graphic: 0.845
mistranslation: 0.726
socket: 0.687
KVM: 0.519
assembly: 0.499
network: 0.411
USB event delivery does not work correctly for macOS guests with XHCI controller without MSI(-X)
Steps to reproduce:
1. Get a macOS VM working. Either on x86-64 with a Q35 machine type, AppleSMC device, and OpenCore bootloader, or on aarch64 using the patch set and instructions linked above.
2. On x86-64, switch to a NEC XHCI controller with MSI and MSI-X support forcibly disabled: `-device nec-usb-xhci,id=xhci,msi=off,msix=off`
3. Boot macOS.
USB events are now extremely laggy. A USB keyboard or mouse becomes almost unusable.
While narrowing down the problem, I established the following facts by experimentation, tracing, and code inspection:
* Although the vmapple platform uses an emulated XHCI PCI device for connecting virtual USB devices, it does not support message-signalled interrupts, in either the MSI or MSI-X persuasion. (This is true in Apple's implementation as well, but the macOS guest's XHCI driver unsurprisingly does work with Apple's PCI/XHCI implementation.)
* macOS guests (and the iBoot bootloader) appear to refuse to drive XHCI controllers with `numintrs < 4`, for both aarch64 and x86-64 architectures. They will generally set up event rings 0, 1, and 2.
* QEMU's PCI XHCI implementation does not appear to implement (as of 9.2.0-rc2) any mitigations for when the controller is used in pin-based IRQ mode. It will happily attempt to use event rings >0 in this case, but interrupts are dropped.
* Linux and FreeBSD guests appear to use only interrupter 0 anyway, so these are not useful references.
It's not entirely clear to me what component is ultimately responsible for the failure here - I suspect there might be some not-quite-right behaviour in both macOS's XHCI driver and Qemu's XHCI implementation, and that these conspire to a non-functional setup.
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