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Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive |
Date: |
Mon, 6 Jul 2015 20:09:21 +0300 |
On Mon, Jul 06, 2015 at 04:21:16PM +0100, Stefan Hajnoczi wrote:
> On Mon, Jul 06, 2015 at 11:32:25AM +0800, Jason Wang wrote:
> >
> >
> > On 07/02/2015 08:46 PM, Stefan Hajnoczi wrote:
> > > On Tue, Jun 30, 2015 at 04:35:24PM +0800, Jason Wang wrote:
> > >> On 06/30/2015 11:06 AM, Fam Zheng wrote:
> > >>> virtio_net_receive still does the check by calling
> > >>> virtio_net_can_receive, if the device or driver is not ready, the packet
> > >>> is dropped.
> > >>>
> > >>> This is necessary because returning false from can_receive complicates
> > >>> things: the peer would disable sending until we explicitly flush the
> > >>> queue.
> > >>>
> > >>> Signed-off-by: Fam Zheng <address@hidden>
> > >>> ---
> > >>> hw/net/virtio-net.c | 1 -
> > >>> 1 file changed, 1 deletion(-)
> > >>>
> > >>> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > >>> index d728233..dbef0d0 100644
> > >>> --- a/hw/net/virtio-net.c
> > >>> +++ b/hw/net/virtio-net.c
> > >>> @@ -1503,7 +1503,6 @@ static int virtio_net_load_device(VirtIODevice
> > >>> *vdev, QEMUFile *f,
> > >>> static NetClientInfo net_virtio_info = {
> > >>> .type = NET_CLIENT_OPTIONS_KIND_NIC,
> > >>> .size = sizeof(NICState),
> > >>> - .can_receive = virtio_net_can_receive,
> > >>> .receive = virtio_net_receive,
> > >>> .link_status_changed = virtio_net_set_link_status,
> > >>> .query_rx_filter = virtio_net_query_rxfilter,
> > >> A side effect of this patch is it will read and then drop packet is
> > >> guest driver is no ok.
> > > I think that the semantics of .can_receive() and .receive() return
> > > values are currently incorrect in many NICs. They have .can_receive()
> > > functions that return false for conditions where .receive() would
> > > discard the packet. So what happens is that packets get queued when
> > > they should actually be discarded.
> >
> > Yes, but they are bugs more or less.
> >
> > >
> > > The purpose of the flow control (queuing) mechanism is to tell the
> > > sender to hold off until the receiver has more rx buffers available.
> > > It's a short-term thing that doesn't included link down, rx disable, or
> > > NIC reset states.
> > >
> > > Therefore, I think this patch will not introduce a regression. It is
> > > adjusting the code to stop queuing packets when they should actually be
> > > dropped.
> > >
> > > Thoughts?
> >
> > I agree there's no functional issue. But it cause wasting of cpu cycles
> > (consider guest is being flooded). Sometime it maybe even dangerous. For
> > tap, we're probably ok since we have 756ae78b but for other backend, we
> > don't.
>
> If the guest uses iptables rules or other mechanisms to drop bogus
> packets the cost is even higher than discarding them at the QEMU layer.
>
> What's more is that if you're using link down as a DoS mitigation
> strategy then you might as well hot unplug the NIC.
>
> Stefan
Frankly, I don't see the point of the patch. Is this supposed to be a
bugfix? If so, there's should be a description about how to trigger the
bug. Is this an optimization? If so there should be some numbers
showing a gain.
--
MST
- Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive, Stefan Hajnoczi, 2015/07/02
- Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive, Jason Wang, 2015/07/05
- Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive, Jason Wang, 2015/07/07
- Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive, Stefan Hajnoczi, 2015/07/08
- Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive, Jason Wang, 2015/07/13