Re: [PATCH v2 2/4] vfio/pci: enable vector on dynamic MSI-X allocation

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 18 Sep 2023 05:45:05 -0400
Jing Liu <jing2.liu@intel.com> wrote:

> The vector_use callback is used to enable vector that is unmasked in
> guest. The kernel used to only support static MSI-X allocation. When
> allocating a new interrupt using "static MSI-X allocation" kernels,
> QEMU first disables all previously allocated vectors and then
> re-allocates all including the new one. The nr_vectors of VFIOPCIDevice
> indicates that all vectors from 0 to nr_vectors are allocated (and may
> be enabled), which is used to to loop all the possibly used vectors
                             ^^ ^^

s/to to/to/

> When, e.g., disabling MSI-X interrupts.
> 
> Extend the vector_use function to support dynamic MSI-X allocation when
> host supports the capability. QEMU therefore can individually allocate
> and enable a new interrupt without affecting others or causing interrupts
> lost during runtime.
> 
> Utilize nr_vectors to calculate the upper bound of enabled vectors in
> dynamic MSI-X allocation mode since looping all msix_entries_nr is not
> efficient and unnecessary.
> 
> Signed-off-by: Jing Liu <jing2.liu@intel.com>
> Tested-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
> Changes since v1:
> - Revise Qemu to QEMU.
> 
> Changes since RFC v1:
> - Test vdev->msix->noresize to identify the allocation mode. (Alex)
> - Move defer_kvm_irq_routing test out and update nr_vectors in a
>   common place before vfio_enable_vectors(). (Alex)
> - Revise the comments. (Alex)
> ---
>  hw/vfio/pci.c | 44 +++++++++++++++++++++++++++-----------------
>  1 file changed, 27 insertions(+), 17 deletions(-)
> 
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 60654ca28ab8..84987e46fd7a 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -470,6 +470,7 @@ static int vfio_msix_vector_do_use(PCIDevice *pdev, unsigned int nr,
>      VFIOPCIDevice *vdev = VFIO_PCI(pdev);
>      VFIOMSIVector *vector;
>      int ret;
> +    int old_nr_vecs = vdev->nr_vectors;

Minor suggestion, it reads slightly better below if this were something
like:

    bool resizing = !!(vdev->nr_vectors < nr + 1);

Then use the bool in place of the nr+1 tests below.  Thanks,

Alex

>  
>      trace_vfio_msix_vector_do_use(vdev->vbasedev.name, nr);
>  
> @@ -512,33 +513,42 @@ static int vfio_msix_vector_do_use(PCIDevice *pdev, unsigned int nr,
>      }
>  
>      /*
> -     * We don't want to have the host allocate all possible MSI vectors
> -     * for a device if they're not in use, so we shutdown and incrementally
> -     * increase them as needed.
> +     * When dynamic allocation is not supported, we don't want to have the
> +     * host allocate all possible MSI vectors for a device if they're not
> +     * in use, so we shutdown and incrementally increase them as needed.
> +     * nr_vectors represents the total number of vectors allocated.
> +     *
> +     * When dynamic allocation is supported, let the host only allocate
> +     * and enable a vector when it is in use in guest. nr_vectors represents
> +     * the upper bound of vectors being enabled (but not all of the ranges
> +     * is allocated or enabled).
>       */
>      if (vdev->nr_vectors < nr + 1) {
>          vdev->nr_vectors = nr + 1;
> -        if (!vdev->defer_kvm_irq_routing) {
> +    }
> +
> +    if (!vdev->defer_kvm_irq_routing) {
> +        if (vdev->msix->noresize && (old_nr_vecs < nr + 1)) {
>              vfio_disable_irqindex(&vdev->vbasedev, VFIO_PCI_MSIX_IRQ_INDEX);
>              ret = vfio_enable_vectors(vdev, true);
>              if (ret) {
>                  error_report("vfio: failed to enable vectors, %d", ret);
>              }
> -        }
> -    } else {
> -        Error *err = NULL;
> -        int32_t fd;
> -
> -        if (vector->virq >= 0) {
> -            fd = event_notifier_get_fd(&vector->kvm_interrupt);
>          } else {
> -            fd = event_notifier_get_fd(&vector->interrupt);
> -        }
> +            Error *err = NULL;
> +            int32_t fd;
>  
> -        if (vfio_set_irq_signaling(&vdev->vbasedev,
> -                                     VFIO_PCI_MSIX_IRQ_INDEX, nr,
> -                                     VFIO_IRQ_SET_ACTION_TRIGGER, fd, &err)) {
> -            error_reportf_err(err, VFIO_MSG_PREFIX, vdev->vbasedev.name);
> +            if (vector->virq >= 0) {
> +                fd = event_notifier_get_fd(&vector->kvm_interrupt);
> +            } else {
> +                fd = event_notifier_get_fd(&vector->interrupt);
> +            }
> +
> +            if (vfio_set_irq_signaling(&vdev->vbasedev,
> +                                       VFIO_PCI_MSIX_IRQ_INDEX, nr,
> +                                       VFIO_IRQ_SET_ACTION_TRIGGER, fd, &err)) {
> +                error_reportf_err(err, VFIO_MSG_PREFIX, vdev->vbasedev.name);
> +            }
>          }
>      }
>