Re: [PULL v4 42/83] virtio-rng-pci: Allow setting nvectors, so we can use MSI-X

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Michael S. Tsirkin (mst@redhat.com) wrote:
> From: David Daney <david.daney@fungible.com>
> 
> Most other virtio-pci devices allow MSI-X, let's have it for rng too.
> 
> Signed-off-by: David Daney <david.daney@fungible.com>
> Reviewed-by: Marcin Nowakowski <marcin.nowakowski@fungible.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@fungible.com>
> Message-Id: <20221014160947.66105-1-philmd@fungible.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

This breaks migration compatibility 7.1->7.2 :

(qemu) qemu: get_pci_config_device: Bad config data: i=0x34 read: 84 device: 98 cmask: ff wmask: 0 w1cmask:0
qemu: Failed to load PCIDevice:config
qemu: Failed to load virtio-rng:virtio
qemu: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-rng'
qemu: load of migration failed: Invalid argument

because the destination is configured with msi-x but the source isn't.

The fix is in theory simple:
diff --git a/hw/core/machine.c b/hw/core/machine.c
index f589b92909..45459d1cef 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -45,6 +45,7 @@ const size_t hw_compat_7_2_len = G_N_ELEMENTS(hw_compat_7_2);
 
 GlobalProperty hw_compat_7_1[] = {
     { "virtio-device", "queue_reset", "false" },
+    { "virtio-rng-pci", "vectors", "0" },
 };
 const size_t hw_compat_7_1_len = G_N_ELEMENTS(hw_compat_7_1);

the gotcha is that will break 7.2->7.2-fixed.

(I guess you can also work around it by explicitly passing vectors=0 to
the virtio-rng on the cli)

Does anyone have preferences as to whether that should be fixed in the
7.2 world or left as is?

This is:
https://bugzilla.redhat.com/show_bug.cgi?id=2155749

Dave

> ---
>  hw/virtio/virtio-rng-pci.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/hw/virtio/virtio-rng-pci.c b/hw/virtio/virtio-rng-pci.c
> index 151ece6f94..6e76f8b57b 100644
> --- a/hw/virtio/virtio-rng-pci.c
> +++ b/hw/virtio/virtio-rng-pci.c
> @@ -13,6 +13,7 @@
>  
>  #include "hw/virtio/virtio-pci.h"
>  #include "hw/virtio/virtio-rng.h"
> +#include "hw/qdev-properties.h"
>  #include "qapi/error.h"
>  #include "qemu/module.h"
>  #include "qom/object.h"
> @@ -31,11 +32,23 @@ struct VirtIORngPCI {
>      VirtIORNG vdev;
>  };
>  
> +static Property virtio_rng_properties[] = {
> +    DEFINE_PROP_BIT("ioeventfd", VirtIOPCIProxy, flags,
> +                    VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT, true),
> +    DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors,
> +                       DEV_NVECTORS_UNSPECIFIED),
> +    DEFINE_PROP_END_OF_LIST(),
> +};
> +
>  static void virtio_rng_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>  {
>      VirtIORngPCI *vrng = VIRTIO_RNG_PCI(vpci_dev);
>      DeviceState *vdev = DEVICE(&vrng->vdev);
>  
> +    if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
> +        vpci_dev->nvectors = 2;
> +    }
> +
>      if (!qdev_realize(vdev, BUS(&vpci_dev->bus), errp)) {
>          return;
>      }
> @@ -54,6 +67,7 @@ static void virtio_rng_pci_class_init(ObjectClass *klass, void *data)
>      pcidev_k->device_id = PCI_DEVICE_ID_VIRTIO_RNG;
>      pcidev_k->revision = VIRTIO_PCI_ABI_VERSION;
>      pcidev_k->class_id = PCI_CLASS_OTHERS;
> +    device_class_set_props(dc, virtio_rng_properties);
>  }
>  
>  static void virtio_rng_initfn(Object *obj)
> -- 
> MST
> 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK