Re: [PATCH RFC v1 2/3] vfio/pci: enable vector on dynamic MSI-X allocation

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

On Thu, 27 Jul 2023 03:24:09 -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
> 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>
> ---
>  hw/vfio/pci.c | 40 +++++++++++++++++++++++++++-------------
>  1 file changed, 27 insertions(+), 13 deletions(-)
> 
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 0c4ac0873d40..8c485636445c 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -512,12 +512,20 @@ 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.
> +     * And nr_vectors stands for the number of vectors being allocated.

"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 stands
> +     * for the upper bound of vectors being enabled (but not all of the
> +     * ranges is allocated or enabled).

s/stands for/represents/

>       */
> -    if (vdev->nr_vectors < nr + 1) {
> +    if ((vdev->msix->irq_info_flags & VFIO_IRQ_INFO_NORESIZE) &&

Testing vdev->msix->noresize would be cleaner.

> +        (vdev->nr_vectors < nr + 1)) {
>          vdev->nr_vectors = nr + 1;
> +
>          if (!vdev->defer_kvm_irq_routing) {
>              vfio_disable_irqindex(&vdev->vbasedev, VFIO_PCI_MSIX_IRQ_INDEX);
>              ret = vfio_enable_vectors(vdev, true);
> @@ -529,16 +537,22 @@ static int vfio_msix_vector_do_use(PCIDevice *pdev, unsigned int nr,
>          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);
> -        }
> +        if (!vdev->defer_kvm_irq_routing) {
> +            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);
> +            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);
> +            }
> +        }
> +        /* Increase for dynamic allocation case. */
> +        if (vdev->nr_vectors < nr + 1) {
> +            vdev->nr_vectors = nr + 1;
>          }

We now have two branches where the bulk of the code is skipped when
defer_kvm_irq_routing is enabled and doing effectively the same update
to nr_vectors otherwise.  This suggests we should move the
defer_kvm_irq_routing test out and create a common place to update
nr_vectors.  Thanks,

Alex