Before kernel v6.5, dynamic allocation of MSI-X interrupts was not supported. Qemu therefore when allocating a new interrupt, should first release all previously allocated interrupts (including disable of MSI-X) and re-allocate all interrupts that includes the new one.
The kernel series [1] adds the support of dynamic MSI-X allocation to vfio-pci and uses the existing flag VFIO_IRQ_INFO_NORESIZE to guide user space, that when dynamic MSI-X is supported the flag is cleared. This series makes the behavior for VFIO PCI devices when dynamic MSI-X allocation is supported. When guest unmasks an interrupt, Qemu can directly allocate an interrupt on host for this and has nothing to do with the previously allocated ones. Therefore, host only allocates interrupts for those unmasked (enabled) interrupts inside guest when dynamic MSI-X allocation is supported by device. During migration restore, Qemu calls vfio_enable_vectors() to enable MSI-X and interrupts. Since the API causes that a number of irqs set to host kernel are all allocated when enabling MSI-X, to avoid this, one possible way is that Qemu first sets vector 0 to host kernel to enable MSI-X with an invalid fd. After MSI-X enabling, the API can decide which should be allocated via the event fd value. In this way, the interrupts allocation on target would be the same as migration source. Jing Liu (2): vfio/pci: enable vector on dynamic MSI-X allocation vfio/pci: dynamic MSI-X allocation in interrupt restoring Reinette Chatre (1): vfio/pci: detect the support of dynamic MSI-X allocation hw/vfio/pci.c | 84 +++++++++++++++++++++++++++++++++++++------- hw/vfio/pci.h | 1 + hw/vfio/trace-events | 2 ++ 3 files changed, 74 insertions(+), 13 deletions(-) -- 2.27.0
