MSI, and disabling it for better passthrough?

[Zarathustra[H]]

Hey, can anyone speak to this?

I have noticed that for many devices to work properly in passthrough, MSI needs to be disabled either guest side or by manually configuring it in the host.

I'm wondering if it just makes sense to as a matter of course disable MSI across the board for all passthroughs.

I'm just not sure what MSI is, and what I might be giving up or hurting by doing so.

Can anyone more knowledgeable than me on this subject speak to this?

 

[Zarathustra[H]]

So,

Some more reading informs me that MSI stands for message signaled interrupt, which replaces traditional electrical pin based interrupts by instead writing a message to a special memory address in the CPU.

As compared to traditional electric pin based interrupts, there are some small performance benefits to MSI.

The part I don't understand is this.

The PCI spec had a pin for interrupts. The PCIe spec doesn't, so presumably it relies on MSI.

If this were the case, disabling MSI would result in the absence of the ability of the device to send the CPU an interrupt, which would be bad, on the level of not working.

This obviously isn't the case though, since prior to Windows Vista, we didn't ahve support for MSI, and PCIe cards worked just fine in Windows XP and earlier.

So I guess my question still stands.


If I disable MSI on all devices I pass through on my ESXi box, is it just a matter of trading some performance for stability? How does the system handle interrupts without MSI, since the PCIe spec doesn't have a pin for interrupt?

I'd appreciate any thoughts on this subject.