Re: [Qemu-devel] [PATCH] vfio-pci: Make host MSI-X enable track guest

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 20, 2012 at 09:06:41AM -0700, Alex Williamson wrote:
> Guests typically enable MSI-X with all of the vectors in the MSI-X
> vector table masked.  Only when the vector is enabled does the vector
> get unmasked, resulting in a vector_use callback.  These two points,
> enable and unmask, correspond to pci_enable_msix() and request_irq()
> for Linux guests.  Some drivers rely on VF/PF or PF/fw communication
> channels that expect the physical state of the device to match the
> guest visible state of the device.  They don't appreciate lazily
> enabling MSI-X on the physical device.
> 
> To solve this, enable MSI-X with a single vector when the MSI-X
> capability is enabled and immediate disable the vector.  This leaves
> the physical device in exactly the same state between host and guest.
> Furthermore, the brief gap where we enable vector 0, it fires into
> userspace, not KVM, so the guest doesn't get spurious interrupts.
> Ideally we could call VFIO_DEVICE_SET_IRQS with the right parameters
> to enable MSI-X with zero vectors, but this will currently return an
> error as the Linux MSI-X interfaces do not allow it.
> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Do you need an interface for this?  Can you do low-level pci config
access instead?  I imagine you would then enable MSIX and mask all
vectors at the same time.

No?

> ---
>  hw/vfio_pci.c |   31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> VFIO makes this a bit cleaner, so I think this is both the stable and
> final fix here.
> 
> diff --git a/hw/vfio_pci.c b/hw/vfio_pci.c
> index 7c27834..5178ccc 100644
> --- a/hw/vfio_pci.c
> +++ b/hw/vfio_pci.c
> @@ -561,8 +561,9 @@ static int vfio_enable_vectors(VFIODevice *vdev, bool msix)
>      return ret;
>  }
>  
> -static int vfio_msix_vector_use(PCIDevice *pdev,
> -                                unsigned int nr, MSIMessage msg)
> +static int vfio_msix_vector_do_use(PCIDevice *pdev, unsigned int nr,
> +                                   MSIMessage msg, bool try_kvm,
> +                                   IOHandler *handler)
>  {
>      VFIODevice *vdev = DO_UPCAST(VFIODevice, pdev, pdev);
>      VFIOMSIVector *vector;
> @@ -586,7 +587,7 @@ static int vfio_msix_vector_use(PCIDevice *pdev,
>       * Attempt to enable route through KVM irqchip,
>       * default to userspace handling if unavailable.
>       */
> -    vector->virq = kvm_irqchip_add_msi_route(kvm_state, msg);
> +    vector->virq = try_kvm ? kvm_irqchip_add_msi_route(kvm_state, msg) : -1;
>      if (vector->virq < 0 ||
>          kvm_irqchip_add_irqfd_notifier(kvm_state, &vector->interrupt,
>                                         vector->virq) < 0) {
> @@ -595,7 +596,7 @@ static int vfio_msix_vector_use(PCIDevice *pdev,
>              vector->virq = -1;
>          }
>          qemu_set_fd_handler(event_notifier_get_fd(&vector->interrupt),
> -                            vfio_msi_interrupt, NULL, vector);
> +                            handler, NULL, vector);
>      }
>  
>      /*
> @@ -638,6 +639,12 @@ static int vfio_msix_vector_use(PCIDevice *pdev,
>      return 0;
>  }
>  
> +static int vfio_msix_vector_use(PCIDevice *pdev,
> +                                unsigned int nr, MSIMessage msg)
> +{
> +    return vfio_msix_vector_do_use(pdev, nr, msg, true, vfio_msi_interrupt);
> +}
> +
>  static void vfio_msix_vector_release(PCIDevice *pdev, unsigned int nr)
>  {
>      VFIODevice *vdev = DO_UPCAST(VFIODevice, pdev, pdev);
> @@ -696,6 +703,22 @@ static void vfio_enable_msix(VFIODevice *vdev)
>  
>      vdev->interrupt = VFIO_INT_MSIX;
>  
> +    /*
> +     * Some communication channels between VF & PF or PF & fw rely on the
> +     * physical state of the device and expect that enabling MSI-X from the
> +     * guest enables the same on the host.  When our guest is Linux, the
> +     * guest driver call to pci_enable_msix() sets the enabling bit in the
> +     * MSI-X capability, but leaves the vector table masked.  We therefore
> +     * can't rely on a vector_use callback (from request_irq() in the guest)
> +     * to switch the physical device into MSI-X mode because that may come a
> +     * long time after pci_enable_msix().  This code enables vector 0 with
> +     * 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.
> +     */
> +    vfio_msix_vector_do_use(&vdev->pdev, 0, (MSIMessage) { 0, 0 }, false, NULL);
> +    vfio_msix_vector_release(&vdev->pdev, 0);
> +
>      if (msix_set_vector_notifiers(&vdev->pdev, vfio_msix_vector_use,
>                                    vfio_msix_vector_release)) {
>          error_report("vfio: msix_set_vector_notifiers failed\n");