Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT

From: Jason Wang
Date: Tue Apr 17 2012 - 06:46:51 EST

On 04/17/2012 06:18 PM, Michael S. Tsirkin wrote:
On Tue, Apr 17, 2012 at 02:30:27PM +0800, Jason Wang wrote:
> On 04/17/2012 02:07 PM, Michael S. Tsirkin wrote:
> >On Tue, Apr 17, 2012 at 01:54:55PM +0800, Jason Wang wrote:
> >>On 04/17/2012 12:57 PM, Michael S. Tsirkin wrote:
> >>>On Tue, Apr 17, 2012 at 11:27:01AM +0800, Jason Wang wrote:
> >>>>On 04/16/2012 09:39 PM, Michael S. Tsirkin wrote:
> >>>>>On Mon, Apr 16, 2012 at 04:28:10PM +0800, Jason Wang wrote:
> >>>>>>> On 04/16/2012 03:16 PM, Michael S. Tsirkin wrote:
> >>>>>>>> >On Mon, Apr 16, 2012 at 02:08:33PM +0800, Jason Wang wrote:
> >>>>>>>>> >>Currently, we restart tx polling unconditionally when sendmsg()
> >>>>>>>>> >>fails. This would cause unnecessary wakeups of vhost wokers as it's
> >>>>>>>>> >>only needed when the socket send buffer were exceeded.
> >>>>>>>> >Why is this a problem?
> >>>>>>> > This issue is when guest driver is able to hit the
> >>>>>>-EFAULT, vhost
> >>>>>>> discard the the descriptor and restart the polling. This would wake
> >>>>>>> vhost thread and repeat the loop again which waste cpu.
> >>>>>Does same thing happen if we get an error from copy from user?
> >>>>>
> >>>>Right, so do you think it makes sense that we only restart polling
> >>>>on -EAGAIN or -ENOBUFS?
> >>>Sounds OK. BTW how do you test this?
> >>>
> >>Not very hard, w/o this patch, we can see almost 100% cpu
> >>utilization for vhost thread if guest hit EFAULT or EINVAL. With
> >>this patch, the cpu utilization should be very low I think.
> >Yes but do you have a test that makes guest hit EFAULT or EINVAL?
> > Looks like we can do this by supplying an invalid hdr_len in vnet
> header as tap does the check for this.
Ah so you patched qemu to do this? Cool. Can you post the patch for testing pls?

No, I mean patch the guest driver like this:

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 019da01..6e2f487 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -582,7 +582,7 @@ static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)

if (skb_is_gso(skb)) {
- hdr->hdr.hdr_len = skb_headlen(skb);
+ hdr->hdr.hdr_len = 65535;
hdr->hdr.gso_size = skb_shinfo(skb)->gso_size;
if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4)
hdr->hdr.gso_type = VIRTIO_NET_HDR_GSO_TCPV4;

btw, If we still choose to drop the packet, we can hit -EFAULT by send a descriptor with a large number of pages.
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