lkml.org 
[lkml]   [2023]   [Mar]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Subject[RFC PATCH 0/8] vfio/pci: Support dynamic allocation of MSI-X interrupts
    Date
    == 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@MWHPR11MB1886.namprd11.prod.outlook.com/#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

    \
     
     \ /
      Last update: 2023-03-27 01:01    [W:7.221 / U:0.036 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site