RE: [PATCH V5 10/11] vfio/pci: Support dynamic MSI-X

[Tian, Kevin]

> From: Chatre, Reinette <reinette.chatre@xxxxxxxxx>
> Sent: Thursday, May 11, 2023 11:45 PM
> 
> 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@xxxxxxxxx>
> Link:
> https://lore.kernel.org/lkml/20230403211841.0e206b67.alex.williamson@re
> dhat.com/

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>