Allow adding MSI capability via vfu_pci_add_capability

[FlorianFreudiger]

Currently vfu_pci_add_capability(...) does not allow adding the MSI capability, only MSIX and a handful of other capabilities, even though the MSI capability definition is already present in include/pci_caps/msi.h.

The new cap_write_msi callback function is copied from cap_write_msix and adjusted to msicap, omitting the "function mask" bit which is not present in MSI's message control.

The size of the MSI capability does not seem to be defined in pci_regs, just like in

libvfio-user/include/pci_caps/msi.h

Line 68 in 8872c36

_Static_assert(sizeof(struct msicap) == 0x18, "bad MSICAP size");

compared to:

libvfio-user/include/pci_caps/msix.h

Line 72 in 8872c36

_Static_assert(sizeof(struct msixcap) == PCI_CAP_MSIX_SIZEOF, "bad MSI-X size");

[jlevon]

Hi Florian, thanks for your contribution! Would you mind looking at adding a pytest for this? Let me know if any questions.

[FlorianFreudiger]

I noticed there are some other fields that will need to be included in cap_write_msi

[FlorianFreudiger]

I also found a small bug in the existing cap_write_msix I copied, it logs the opposite MSI-X enable bit.

libvfio-user/lib/pci_caps.c

Line 195 in 8872c36

msix->mxc.mxe ? "enable" : "disable");

it should either check new_msix.mxc.mxe instead of msix->mxc.mxe or the assignment should be performed before the log, similar to the other write functions.

@jlevon Let me know if I should include the small fix in this PR.

[jlevon]

I also found a small bug in the existing cap_write_msix I copied, it logs the opposite MSI-X enable bit.

libvfio-user/lib/pci_caps.c

Line 195 in 8872c36

msix->mxc.mxe ? "enable" : "disable");

it should either check new_msix.mxc.mxe instead of msix->mxc.mxe or the assignment should be performed before the log, similar to the other write functions.

@jlevon Let me know if I should include the small fix in this PR.

good spot! Please open a separate PR for that fix, thank you!

[jlevon]

thanks! Just one tiny nit

[jlevon]

@FlorianFreudiger please rebase and I'll merge