vmm(4), bhyve(8): Expose kernel-emulated special devices to userspace

[cem]

Summary

Expose the special kernel LAPIC, IOAPIC, and HPET devices to userspace
for use in, e.g., fallback instruction emulation (when userspace has a
newer instruction decode/emulation layer than the kernel vmm(4)).

Plumb the ioctl through libvmmapi and register the memory ranges in
bhyve(8).

[cem]

Don't panic the kernel if the user-supplied address range isn't in a known range.

[grehan]

sys/amd64/include/vmm_dev.h

274

This needs to be added to the capability list in vmmapi.c:vm_get_ioctls()

usr.sbin/bhyve/apic.c

1 ↗

(On Diff #70860)

minor naming nit: maybe kernemu_dev.c or similar ?

[cem]

Thanks for taking a look! Does this approach seem plausibly reasonable, outside of the implementation-detail type issues already raised? Without it or something like it, I don't think we can really emulate instructions in userspace. I first looked at using the existing memory interface, but ordinary reads and writes have no way to specify the accessing vcpu, and the specific vcpu is essential for these magic regions.

sys/amd64/include/vmm_dev.h

274

Thanks, I missed that.

Other similar ioctls seem to have GET/SET separate versions, but I'm not sure it buys us anything here. Are you ok with the combined get/setter ioctl, or should I split this one up as well?

usr.sbin/bhyve/apic.c

1 ↗

(On Diff #70860)

Sure, that works for me. I'll rename the _init routine to correspond.

[grehan]

Thanks for taking a look! Does this approach seem plausibly reasonable, outside of the implementation-detail type issues already raised?

Yep, I think it's ok.

Other similar ioctls seem to have GET/SET separate versions, but I'm not sure it buys us anything here.
Are you ok with the combined get/setter ioctl, or should I split this one up as well?

Hmmm, if you're going to revisit the code anyway, it might be worth splitting that out for consistency.

[cem]

• Split into two ioctls
• Rename bhyve component s/apic/kernemu_dev/
• Add ioctls to vm_get_ioctls()