Re: [PATCH v2 1/3] virtio_net: Stop doing DMA from the stack

[Andy Lutomirski]

On Wed, Oct 28, 2015 at 12:07 AM, Michael S. Tsirkin  wrote:
> How about this instead? Less code, more robust.
>
> Warning: untested.  If you do like this approach, Tested-by would be
> appreciated.

I like it.

Tested-by: Andy Lutomirski 

--Andy
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Re: [PATCH v2 1/3] virtio_net: Stop doing DMA from the stack

[Michael S. Tsirkin]

On Tue, Oct 27, 2015 at 10:30:19PM -0700, Andy Lutomirski wrote:
> From: Andy Lutomirski 
> 
> Once virtio starts using the DMA API, we won't be able to safely DMA
> from the stack.  virtio-net does a couple of config DMA requests
> from small stack buffers -- switch to using dynamically-allocated
> memory.
> 
> This should have no effect on any performance-critical code paths.
> 
> Cc: net...@vger.kernel.org
> Cc: "Michael S. Tsirkin" 
> Cc: virtualizat...@lists.linux-foundation.org
> Reviewed-by: Joerg Roedel 
> Signed-off-by: Andy Lutomirski 
> ---
> 
> Hi Michael and DaveM-
> 
> This is a prerequisite for the virtio DMA fixing project.  It works
> as a standalone patch, though.  Would it make sense to apply it to
> an appropriate networking tree now?
> 
>  drivers/net/virtio_net.c | 53 
> 
>  1 file changed, 36 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index d8838dedb7a4..4f10f8a58811 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -976,31 +976,43 @@ static bool virtnet_send_command(struct virtnet_info 
> *vi, u8 class, u8 cmd,
>struct scatterlist *out)
>  {
>   struct scatterlist *sgs[4], hdr, stat;
> - struct virtio_net_ctrl_hdr ctrl;
> - virtio_net_ctrl_ack status = ~0;
> +
> + struct {
> + struct virtio_net_ctrl_hdr ctrl;
> + virtio_net_ctrl_ack status;
> + } *buf;
> +
>   unsigned out_num = 0, tmp;
> + bool ret;
>  
>   /* Caller should know better */
>   BUG_ON(!virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_VQ));
>  
> - ctrl.class = class;
> - ctrl.cmd = cmd;
> + buf = kmalloc(sizeof(*buf), GFP_ATOMIC);
> + if (!buf)
> + return false;

This is problematic. The command is never retried, the error
is propagated to userspace.

> + buf->status = ~0;
> +
> + buf->ctrl.class = class;
> + buf->ctrl.cmd = cmd;
>   /* Add header */
> - sg_init_one(, , sizeof(ctrl));
> + sg_init_one(, >ctrl, sizeof(buf->ctrl));
>   sgs[out_num++] = 
>  
>   if (out)
>   sgs[out_num++] = out;
>  
>   /* Add return status. */
> - sg_init_one(, , sizeof(status));
> + sg_init_one(, >status, sizeof(buf->status));
>   sgs[out_num] = 
>  
>   BUG_ON(out_num + 1 > ARRAY_SIZE(sgs));
>   virtqueue_add_sgs(vi->cvq, sgs, out_num, 1, vi, GFP_ATOMIC);
>  
> - if (unlikely(!virtqueue_kick(vi->cvq)))
> - return status == VIRTIO_NET_OK;
> + if (unlikely(!virtqueue_kick(vi->cvq))) {
> + ret = (buf->status == VIRTIO_NET_OK);
> + goto out;
> + }
>  
>   /* Spin for a response, the kick causes an ioport write, trapping
>* into the hypervisor, so the request should be handled immediately.
> @@ -1009,7 +1021,11 @@ static bool virtnet_send_command(struct virtnet_info 
> *vi, u8 class, u8 cmd,
>  !virtqueue_is_broken(vi->cvq))
>   cpu_relax();
>  
> - return status == VIRTIO_NET_OK;
> + ret = (buf->status == VIRTIO_NET_OK);
> +
> +out:
> + kfree(buf);
> + return ret;
>  }
>  
>  static int virtnet_set_mac_address(struct net_device *dev, void *p)
> @@ -1151,7 +1167,7 @@ static void virtnet_set_rx_mode(struct net_device *dev)
>  {
>   struct virtnet_info *vi = netdev_priv(dev);
>   struct scatterlist sg[2];
> - u8 promisc, allmulti;
> + u8 *cmdbyte;
>   struct virtio_net_ctrl_mac *mac_data;
>   struct netdev_hw_addr *ha;
>   int uc_count;
> @@ -1163,22 +1179,25 @@ static void virtnet_set_rx_mode(struct net_device 
> *dev)
>   if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_RX))
>   return;
>  
> - promisc = ((dev->flags & IFF_PROMISC) != 0);
> - allmulti = ((dev->flags & IFF_ALLMULTI) != 0);
> + cmdbyte = kmalloc(sizeof(*cmdbyte), GFP_ATOMIC);
> + if (!cmdbyte)
> + return;

Here the error is ignored, rx mode will be incorrect.
OTOH it looks like that's already the case.

>  
> - sg_init_one(sg, , sizeof(promisc));
> + sg_init_one(sg, cmdbyte, sizeof(*cmdbyte));
>  
> + *cmdbyte = ((dev->flags & IFF_PROMISC) != 0);
>   if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_RX,
> VIRTIO_NET_CTRL_RX_PROMISC, sg))
>   dev_warn(>dev, "Failed to %sable promisc mode.\n",
> -  promisc ? "en" : "dis");
> -
> - sg_init_one(sg, , sizeof(allmulti));
> +  *cmdbyte ? "en" : "dis");
>  
> + *cmdbyte = ((dev->flags & IFF_ALLMULTI) != 0);
>   if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_RX,
> VIRTIO_NET_CTRL_RX_ALLMULTI, sg))
>   dev_warn(>dev, "Failed to %sable allmulti mode.\n",
> -  allmulti ? "en" : "dis");
> +