pci_msix_alloc_irq_at() enables an individual MSI-X interrupt to be
allocated after MSI-X enabling.

Use dynamic MSI-X (if supported by the device) to allocate an interrupt
after MSI-X is enabled. An MSI-X interrupt is dynamically allocated at
the time a valid eventfd is assigned. This is different behavior from
a range provided during MSI-X enabling where interrupts are allocated
for the entire range whether a valid eventfd is provided for each
interrupt or not.

The PCI-MSIX API requires that some number of irqs are allocated for
an initial set of vectors when enabling MSI-X on the device. When
dynamic MSIX allocation is not supported, the vector table, and thus
the allocated irq set can only be resized by disabling and re-enabling
MSI-X with a different range. In that case the irq allocation is
essentially a cache for configuring vectors within the previously
allocated vector range. When dynamic MSI-X allocation is supported,
the API still requires some initial set of irqs to be allocated, but
also supports allocating and freeing specific irq vectors both
within and beyond the initially allocated range.

For consistency between modes, as well as to reduce latency and improve
reliability of allocations, and also simplicity, this implementation
only releases irqs via pci_free_irq_vectors() when either the interrupt
mode changes or the device is released.
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/lkml/20230403211841.0e206b67.alex.williamson@redhat.com/Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/956c47057ae9fd45591feaa82e9ae20929889249.1683740667.git.reinette.chatre@intel.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>