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[vfio-users] Use MSI or not?

I have 2 windows instances running with GPU (nvidia gtx) passthrough via vfio. Initially when I started, I had to enable MSI or guest performance was absolutely horrible.

I’ve recently done the full upgrade to Ubuntu 16 including the newest 4.4 kernel, etc.

So somewhere between updating the host kernel, and updating the NVIDIA drivers on the guest… MSI got disabled and my guests, and performance was fine (possibly even less %sys than when using MSI).

Is there a really good reason to use MSI? I also noticed that the devices seem to be using fasteoi emulation where it wasnt before (use to list the devices with plain old interrupt assignment. Now both cards seem to share a total of 2 fasteoi interrupts:

IR-IO-APIC   16-fasteoi   vfio-intx(0000:01:00.0), vfio-intx(0000:06:00.0)

  17:         53         58          0          0  IR-IO-APIC   17-fasteoi   vfio-intx(0000:01:00.1), vfio-intx(0000:06:00.1)

The only other thing I can think of that changed is that I’m forcing the guests to use x2apic. I’ve read that x2apic with APICv hardware support offers the best virtual interrupt handling available in KVM/QEMU. Is it possible that this is even better than virtual MSI?

Basically, I was wondering if somebody could explain the relationship/differences between fastEOI/MSI/x2APIC in the context of 4.4+ linux and modern intel hardware (skylake in this case).

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