msi: report error for attempt to use CPU > 255

[emaste]

Summary

The MSI/MSI-X address includes 8 bits to encode the Destination ID (CPU/APIC ID). Previously IDs over 255 overlapped with the fixed portion of the address, resulting in an invalid value. Instead, print an error message and return EINVAL.

[emaste]

My hope here is to provide a bread-crumb if anyone tries running on systems with high CPU counts. Ideally I think this would become a KASSERT, with higher levels either relying on IOMMU or not assigning to CPUs > 255.

[jhb]

I suggested something like this on IRC yesterday, yes. I think it's fine to just fail here rather than panicking as it is a bit nicer to the user.

sys/x86/x86/msi.c

634

[emaste]

sys/x86/x86/msi.c

634

I was a bit surprised that it was msi_cpu, is this historical leftovers? (I.e., would we use msi_apic_id if writing this anew?)