== Cover letter == Qemu allocates interrupts incrementally at the time the guest unmasks an interrupt, for example each time a Linux guest runs request_irq(). Dynamic allocation of MSI-X interrupts was not possible until v6.2 [1]. This prompted Qemu to, when allocating a new interrupt, first release all previously allocated interrupts (including disable of MSI-X) followed by re-allocation of all interrupts that includes the new interrupt. Please see [2] for a detailed discussion about this issue. Releasing and re-allocating interrupts may be acceptable if all interrupts are unmasked during device initialization. If unmasking of interrupts occur during runtime this may result in lost interrupts. For example, consider an accelerator device with multiple work queues, each work queue having a dedicated interrupt. A work queue can be enabled at any time with its associated interrupt unmasked while other work queues are already active. Having all interrupts released and MSI-X disabled to enable the new work queue will impact active work queues. This series builds on the recent interrupt sub-system core changes that added support for dynamic MSI-X allocation after initial MSI-X enabling. Add support for dynamic MSI-X allocation to vfio-pci. A flag indicating lack of support for dynamic allocation already exist: VFIO_IRQ_INFO_NORESIZE and has always been set for MSI and MSI-X. With support for dynamic MSI-X the flag is cleared for MSI-X, enabling Qemu to modify its behavior. == Why is this an RFC ? == vfio support for dynamic MSI-X needs to work with existing user space as well as upcoming user space that takes advantage of this feature. I would appreciate guidance on the expectations and requirements surrounding error handling when considering existing user space. For example, consider the following scenario: Start: Consider a passthrough device that supports 10 MSI-X interrupts (0 to 9) and existing Qemu allocated interrupts 0 to 4. Scenario: Qemu (hypothetically) attempts to associate a new action to interrupts 0 to 7. Early checking of this range in vfio_set_irqs_validate_and_prepare() will pass since it is a valid range for the device. What happens after the early checking is considered next: Current behavior (before this series): Since the provided range, 0 - 7, exceeds the allocated range, no action will be taken on existing allocated interrupts 0 - 4 and the Qemu request will fail without making any state changes. New behavior (with this series): Since vfio supports dynamic MSI-X new interrupts will be allocated for vectors 5, 6, and 7. Please note that this changes the behavior encountered by unmodified Qemu: new interrupts are allocated instead of returning an error. Even more, since the range provided by Qemu spans 0 - 7, a failure during allocation of 5, 6, or 7 will result in release of entire range. This series aims to maintain existing error behavior for MSI (please see "vfio/pci: Remove interrupt context counter") but I would appreciate your guidance on whether existing error behavior should be maintained for MSI-X and how to do so if it is a requirement. Any feedback is appreciated Reinette [1] commit 34026364df8e ("PCI/MSI: Provide post-enable dynamic allocation interfaces for MSI-X") [2] https://lore.kernel.org/kvm/MWHPR11MB188603D0D809C1079F5817DC8C099@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/#t Reinette Chatre (8): vfio/pci: Consolidate irq cleanup on MSI/MSI-X disable vfio/pci: Remove negative check on unsigned vector vfio/pci: Prepare for dynamic interrupt context storage vfio/pci: Use xarray for interrupt context storage vfio/pci: Remove interrupt context counter vfio/pci: Move to single error path vfio/pci: Support dynamic MSI-x vfio/pci: Clear VFIO_IRQ_INFO_NORESIZE for MSI-X drivers/vfio/pci/vfio_pci_core.c | 3 +- drivers/vfio/pci/vfio_pci_intrs.c | 376 ++++++++++++++++++++++-------- include/linux/vfio_pci_core.h | 3 +- include/uapi/linux/vfio.h | 3 + 4 files changed, 286 insertions(+), 99 deletions(-) base-commit: eeac8ede17557680855031c6f305ece2378af326 -- 2.34.1