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

[Jason Wang]

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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