Re: [PATCH] virtio_pci: properly clean up MSI-X state when initialization fails

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

On Sun, Sep 14, 2014 at 08:23:26PM -0700, Anthony Liguori wrote:
> From: Anthony Liguori <aliguori@amazon.com>
> 
> If MSI-X initialization fails after setting msix_enabled = 1, then
> the device is left in an inconsistent state.  This would normally
> only happen if there was a bug in the device emulation but it still
> should be handled correctly.

This might happen if host runs out of resources when trying
to map VQs to vectors, so doesn't have to be a bug.

But I don't see what the problem is:
msix_used_vectors reflects the number of used vectors
and msix_enabled is set, thus vp_free_vectors
will free all IRQs and then disable MSIX.

Where is the inconsistency you speak about?


> 
> Cc: Matt Wilson <msw@amazon.com>
> Cc: Rusty Russell <rusty@rustcorp.com.au>
> Cc: Michael Tsirkin <mst@redhat.com>
> Signed-off-by: Anthony Liguori <aliguori@amazon.com>
> ---
>  drivers/virtio/virtio_pci.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
> index 9cbac33..3d2c2a5 100644
> --- a/drivers/virtio/virtio_pci.c
> +++ b/drivers/virtio/virtio_pci.c
> @@ -357,7 +357,7 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
>  	v = ioread16(vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
>  	if (v == VIRTIO_MSI_NO_VECTOR) {
>  		err = -EBUSY;
> -		goto error;
> +		goto error_msix_used;
>  	}
>  
>  	if (!per_vq_vectors) {
> @@ -369,11 +369,15 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
>  				  vp_vring_interrupt, 0, vp_dev->msix_names[v],
>  				  vp_dev);
>  		if (err)
> -			goto error;
> +			goto error_msix_used;
>  		++vp_dev->msix_used_vectors;
>  	}
>  	return 0;
> +error_msix_used:
> +	v = --vp_dev->msix_used_vectors;
> +	free_irq(vp_dev->msix_entries[v].vector, vp_dev);
>  error:
> +	vp_dev->msix_enabled = 0;

As far as I can see, if you do this, guest will not call
pci_disable_msix thus leaving the device with MSIX enabled.
I'm not sure this won't break drivers if they then 
try to use the device without MSIX, and it
definitely seems less elegant than reverting the
device to the original state.


>  	vp_free_vectors(vdev);
>  	return err;
>  }
> -- 
> 1.7.9.5