xen: automatically disable MSI-X interrupt migration

[royger]

Summary

If the hypervisor version is smaller than 4.6.0. Xen commits 74fd00 and
70a3cb are required on the hypervisor side for this to be fixed, and those
are only included in 4.6.0, so stay on the safe side and disable MSI-X
interrupt migration on anything older than 4.6.0.

It should not cause major performance degradation unless a lot of MSI-X
interrupts are allocated.

Sponsored by: Citrix Systems R&D

Test Plan

@cperciva: could you give this a try on EC2? So far I've only compile tested
it due to the lack of hardware on my side.

[jhb]

Jeremiah (from the original thread) might be able to test this as developed the original patch. I think it only triggers in EC2 when you are using VF's from the Intel 10G adapter.

sys/x86/x86/msi.c

163 ↗

(On Diff #18205)

This value isn't used if the pointer is set and can be left as zero (that is what we normally leave it at for sysctls backed by a variable).

sys/x86/xen/hvm.c

156 ↗

(On Diff #18205)

Perhaps s/in order// to make the message slightly more concise.

[cperciva]

sys/x86/x86/msi.c

152 ↗

(On Diff #18205)

I'd change this to /** in order to mark the comment as non-reformatable. (Like /*- but in the first column that's reserved for marking licenses.)

sys/x86/xen/hvm.c

146 ↗

(On Diff #18205)

Are there no [123].[6789] versions of Xen? This comparison would get them wrong.

[royger]

Fix comments.

[royger]

I'm currently running a tinderbox with it and will commit unless anything comes up.

sys/x86/xen/hvm.c

146 ↗

(On Diff #18205)

Right, what a logic fail...

[royger]

Fix comment.