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Re: [vfio-users] Intel x2APIC and APIC Virtualization (APICv or vAPIC)

My 100 series skylake / i5 6600k doesn't support apicv and used to have x2apic off by default. A recent bios update however switched x2apic on whilst also (finally) providing the cpu firmware bug fix. Consistent with the claims in this thread however I found I have to add nox2apic on my kernel command line to get client benchmarks back to previous values and although the impact is not large - I detected it because it was noticeable under heavy load such as playing games. Haven't looked into why and the stats don't show much change in my collectd history with/without x2apic so I think I will go with the explanation that kvm just does it better. Seems reasonable enough ... although of course it is just as likely to be a bios bug.

Worth bearing in mind in case anybody else finds a similar present that isn't included in the changelog when they do their next bios update. No ACS support on my root port for an i5-6600k but worth bearing in mind that the spec. from intel for the 6th generation does not promise ACS support on the PEG root bridge unless certain circumstanes are fulfilled (mine can provide an ACSViolation error .. no idea if it's useful though). If anybody really wants to know more I'll go find the reference, but I remember it making sense and being because graphics cards are about as easy to isolate for absolute sure as a forest fire. This applies only to v3 compatible cards as I've never seen a v3 compatible card that wasn't just a legacy card with some of the rough edges taken off to make it sufficient for the v3 backwards compatibility requirement. I'm not even sure if there is such a thing as a v3 graphics card - but I'm sure it would isolate fine or not be v3.


On 14/06/16 10:07, Zycorax Tokoroa wrote:
I'd say that at least some consumer product support apicv, as it happens with PCIe ACS probably. My i7-5930K reports it HAS support for it, and it is enabled by default.

Zycorax Tokoroa

cat /sys/module/kvm_intel/parameters/enable_apicv

Should its possible to determine that consumer platforms doesn't
supports APICv at all, I suppose that you may want to add again hv-vapic
for your Windows VMs if your removed it thinking that your platform
supports APICv. I suppose that the performance scale for VMs goes like
this: Emulated x2APIC -> HyperV hv-vapic (For Windows VMs) -> Intel
APICv (On Intel enterprise platforms).

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