Re: [RFC PATCH v2 3/3] vfio: Avoid disabling and enabling vectors repeatedly in VFIO migration

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

On Wed, 9 Dec 2020 16:09:19 +0800
Shenming Lu <lushenming@huawei.com> wrote:

> Different from the normal situation when the guest starts, we can
> know the max unmasked vetctor (at the beginning) after msix_load()
> in VFIO migration. So in order to avoid ineffectively disabling and

s/ineffectively/inefficiently/?  It's "effective" either way I think.

> enabling vectors repeatedly, let's allocate all needed vectors first
> and then enable these unmasked vectors one by one without disabling.
> 
> Signed-off-by: Shenming Lu <lushenming@huawei.com>
> ---
>  hw/pci/msix.c         | 17 +++++++++++++++++
>  hw/vfio/pci.c         | 10 ++++++++--
>  include/hw/pci/msix.h |  2 ++
>  3 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/pci/msix.c b/hw/pci/msix.c
> index 67e34f34d6..bf291d3ff8 100644
> --- a/hw/pci/msix.c
> +++ b/hw/pci/msix.c
> @@ -557,6 +557,23 @@ unsigned int msix_nr_vectors_allocated(const PCIDevice *dev)
>      return dev->msix_entries_nr;
>  }
>  
> +int msix_get_max_unmasked_vector(PCIDevice *dev)
> +{
> +    int max_unmasked_vector = -1;
> +    int vector;
> +
> +    if ((dev->config[dev->msix_cap + MSIX_CONTROL_OFFSET] &
> +        (MSIX_ENABLE_MASK | MSIX_MASKALL_MASK)) == MSIX_ENABLE_MASK) {
> +        for (vector = 0; vector < dev->msix_entries_nr; vector++) {
> +            if (!msix_is_masked(dev, vector)) {
> +                max_unmasked_vector = vector;
> +            }
> +        }
> +    }
> +
> +    return max_unmasked_vector;
> +}

Comments from QEMU PCI folks?

> +
>  static int msix_set_notifier_for_vector(PCIDevice *dev, unsigned int vector)
>  {
>      MSIMessage msg;
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 51dc373695..e755ed2514 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -568,6 +568,9 @@ static void vfio_msix_vector_release(PCIDevice *pdev, unsigned int nr)
>  
>  static void vfio_msix_enable(VFIOPCIDevice *vdev)
>  {
> +    int max_unmasked_vector = msix_get_max_unmasked_vector(&vdev->pdev);
> +    unsigned int used_vector = MAX(max_unmasked_vector, 0);
> +

The above PCI function could also be done inline here pretty easily too:

unsigned int nr, max_vec = 0;

if (!msix_masked(&vdev->pdev))
    for (nr = 0; nr < msix_nr_vectors_allocated(&vdev->pdev); nr++) {
        if (!msix_is_masked(&vdev->pdev, nr)) {
            max_vec = nr;
        }
    }
}

It's a bit cleaner than the msix utility function, imo. 

>      vfio_disable_interrupts(vdev);
>  
>      vdev->msi_vectors = g_new0(VFIOMSIVector, vdev->msix->entries);
> @@ -586,9 +589,12 @@ static void vfio_msix_enable(VFIOPCIDevice *vdev)
>       * triggering to userspace, then immediately release the vector, leaving
>       * the physical device with no vectors enabled, but MSI-X enabled, just
>       * like the guest view.
> +     * If there are unmasked vectors (such as in migration) which will be
> +     * enabled soon, we can allocate them here to avoid ineffectively disabling
> +     * and enabling vectors repeatedly later.

It just happens that migration triggers this usage model where the
MSI-X enable bit is set with vectors unmasked in the vector table, but
this is not unique to migration, guests can follow this pattern as well.
Has this been tested with a variety of guests?  Logically it seems
correct, but always good to prove so.  Thanks,

Alex

>       */
> -    vfio_msix_vector_do_use(&vdev->pdev, 0, NULL, NULL);
> -    vfio_msix_vector_release(&vdev->pdev, 0);
> +    vfio_msix_vector_do_use(&vdev->pdev, used_vector, NULL, NULL);
> +    vfio_msix_vector_release(&vdev->pdev, used_vector);
>  
>      if (msix_set_vector_notifiers(&vdev->pdev, vfio_msix_vector_use,
>                                    vfio_msix_vector_release, NULL)) {
> diff --git a/include/hw/pci/msix.h b/include/hw/pci/msix.h
> index 4c4a60c739..4bfb463fa6 100644
> --- a/include/hw/pci/msix.h
> +++ b/include/hw/pci/msix.h
> @@ -23,6 +23,8 @@ void msix_uninit_exclusive_bar(PCIDevice *dev);
>  
>  unsigned int msix_nr_vectors_allocated(const PCIDevice *dev);
>  
> +int msix_get_max_unmasked_vector(PCIDevice *dev);
> +
>  void msix_save(PCIDevice *dev, QEMUFile *f);
>  void msix_load(PCIDevice *dev, QEMUFile *f);
>