[vfio-users] APIC/IRQ best practises for performance?

[Mario Goebbels]

Hi,

in regards to running a Windows VM with GPU passthrough, what's the best
practise to deal with the (virtual) APIC on an Intel CPU?

Initially, I naively figured that enabling all Hyper-V enlightenments
should end up in best performance, but digging a little more into it, it
doesn't appear so. Actually more the contrary. hv_vapic seems to cause way
more VM exits than APICv, using the new SynICs with kernel 4.5 seems to
disable APICv altogether, and APICv seems only to work when x2APIC is
enabled, which my BIOS however requests to be disabled for some reason (can
be overridden with a kernel parameter).

I suppose from all this and how things are currently, I want APICv to be
functional, right? If so, what issues may force-enabling x2APIC incur?

Thanks for any info,
-mg
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