[Xen-devel] PCI MSI questions

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

Looking at the MSI implementation I have a couple of questions:

1) There currently seems to be a hidden requirement of NR_PIRQS in the
kernel needing to be no smaller than NR_IRQS in the hypervisor.
Otherwise, the pirq returned from PHYSDEVOP_map_pirq may collide
with the dynamic IRQs in the kernel or even be out of range altogether.
Therefore I think that NR_PIRQS has to become a variable defaulting
to 256 but getting initialized from a hypervisor reported value (perhaps
in start_info, or else from a new (sub-)hypercall).

2) While pci_restore_msi_state() properly checks the success of
msi_map_pirq_to_vector(), pci_restore_msix_state() doesn't. Is this
for a reason, or just because the code would get more complex if the
error needs to be handled?

3) The type parameter of xc_physdev_map_pirq{,_msi}() seems
superfluous, or is there any reason why these could be called with the
respectively reversed types?

4) The hypervisor option "msi_irq_enable" seems to be named pretty
oddly - both the "irq" and the "enable" in the name are more or less
redundant. So unless there's a reason for this long a name for an
option that generally I would expect most people want to set (at
least on bigger systems), I'd like to change it into "msi" or, if that's
considered prone for ambiguity, "pci-msi". Also, are there any plans
when to make have default be on rather than off?

Thanks, Jan


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