Re: [Qemu-devel] [PATCH] virtio-serial: Allow one MSI-X vector per virtqueue

[Amit Shah <amit.shah@redhat.com>]

On (Fri) 16 Dec 2011 [09:14:26], zanghongyong@huawei.com wrote:
> From: Hongyong Zang <zanghongyong@huawei.com>
> 
> In pci_enable_msix(), the guest's virtio-serial driver tries to set msi-x
> with one vector per queue. But it fails and eventually all virtio-serial
> ports share one MSI-X vector. Because every virtio-serial port has *two*
> virtqueues, virtio-serial needs (port+1)*2 vectors other than (port+1).

Ouch, good catch.

One comment below:

> This patch allows every virtqueue to have its own MSI-X vector.
> (When the MSI-X vectors needed are more than MSIX_MAX_ENTRIES defined in
> qemu: msix.c, all the queues still share one MSI-X vector as before.)
> 
> Signed-off-by: Hongyong Zang <zanghongyong@huawei.com>
> ---
>  hw/virtio-pci.c |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
> 
> diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
> index 77b75bc..2c9c6fb 100644
> --- a/hw/virtio-pci.c
> +++ b/hw/virtio-pci.c
> @@ -718,8 +718,11 @@ static int virtio_serial_init_pci(PCIDevice *pci_dev)
>          return -1;
>      }
>      vdev->nvectors = proxy->nvectors == DEV_NVECTORS_UNSPECIFIED
> -                                        ? proxy->serial.max_virtserial_ports + 1
> +                                        ? (proxy->serial.max_virtserial_ports + 1) * 2
>                                          : proxy->nvectors;
> +    /*msix.c: #define MSIX_MAX_ENTRIES 32*/
> +    if (vdev->nvectors > 32)
> +        vdev->nvectors = 32;

This change isn't needed: if the proxy->nvectors value exceeds the max
allowed, virtio_init_pci() will end up using a shared vector instead
of separate ones.

Thanks,

		Amit