bhyve: emulate reads of MSI-X capabilities for passthru devices

[corvink]

Summary

Reads of the MSI-X capabilites aren't emulated by passthru devices
yet. The guest will read the host MSI-X capabilites which could
cause issues.

Note: You may wanna have a look at https://github.com/Beckhoff/freebsd-src/commits/phab/corvink/msix-read

[manu]

LGMT.
Will commit next monday unless someone have more comments until then.

[markj]

For my own edification, what problem arises when we go straight to the device? Since writes to the msi-x cap are emulated, I guess a guest might observe that a previous write didn't update some field in the capability, but I'm curious about the details.

usr.sbin/bhyve/pci_passthru.c

747–748

Long line.

[corvink]

In D32686#747013, @markj wrote:

For my own edification, what problem arises when we go straight to the device? Since writes to the msi-x cap are emulated, I guess a guest might observe that a previous write didn't update some field in the capability, but I'm curious about the details.

In the worst case, the guest notices that something goes wrong and stops using the device. I've noticed this mistake while investigating some PCI passthrough issues. It wasn't the root cause of my issues. So, I'm unsure whether there are some real faults caused by the missing emulation of MSI-X reads or not. However, we should correct this mistake.

[corvink]

Shorten long line