Re: [PATCHv2] vhost-net: add dhclient work-around from userspace

From: Anthony Liguori
Date: Wed Jun 30 2010 - 19:24:41 EST

On 06/30/2010 05:31 PM, Michael S. Tsirkin wrote:
On Wed, Jun 30, 2010 at 05:08:11PM -0500, Anthony Liguori wrote:
On 06/29/2010 08:04 AM, Michael S. Tsirkin wrote:
On Tue, Jun 29, 2010 at 12:36:47AM -0700, David Miller wrote:
From: "Michael S. Tsirkin"<mst@xxxxxxxxxx>
Date: Mon, 28 Jun 2010 13:08:07 +0300

Userspace virtio server has the following hack
so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets,
and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps
that use recvmsg with AF_PACKET sockets, don't know how to
The interface to return the relevant information was added
in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient,
so we add a work-around just for DHCP.

Don't bother applying the hack to IPv6 as userspace virtio does not
have a work-around for that - let's hope guests will do the right
thing wrt IPv6.

Signed-off-by: Michael S. Tsirkin<mst@xxxxxxxxxx>
Yikes, this is awful too.

Nothing in the kernel should be mucking around with procotol packets
like this by default. In particular, what the heck does port 67 mean?
Locally I can use it for whatever I want for my own purposes, I don't
have to follow the conventions for service ports as specified by the

But I can't have the packet checksum state be left alone for port 67
traffic on a box using virtio because you have this hack there.

And yes it's broken on machines using the qemu thing, but at least the
hack there is restricted to userspace.
Yes, and I think it was a mistake to add the hack there. This is what
prevented applications from using the new interface in the 3 years
since it was first introduced.
It's far more complicated than that. dhclient is part of ISC's DHCP
package. They do not have a public SCM and instead require you to
join their Software Guild to get access to it.

This problem was identified in one distribution and the patch was
pushed upstream but because they did not have a public SCM, most
other distributions did not see the fix until it appeared in a
release. ISC has a pretty long release cycle historically.

ISC's had the fix for a long time but there was a 3-year gap in
their releases and since their SCM isn't public, users are stuck
with the last release.

This hack makes sense in QEMU as we have a few hacks like this to
fix broken guests.
A primary use of virtualization is to run old
applications so it makes sense for us to do that.
IMO it was wrong to put it in qemu: originally, if a distro shipped
a broken virtio/dhclient combo, it was it's own bug to fix.
But now that qemu has shipped the work-around for so long,
broken guests seemed work.

The guests were broken before qemu implemented this.

virtio-net had checksum offload long before it was ever implemented in qemu. Not even lguest implemented it because the interfaces weren't available in tun/tap. I'm not sure how Rusty ever tested it. We only discovered this bug after checksum offload was implemented in tun/tap and we were able to enable it in QEMU. At that point, the guests had shipped and were in the wild.

The real problem was that we implemented a feature in a guest driver without having a backend implementation and then shipped the code. Shipping untested code is a recipe for failure.

If we had implemented the front-end feature only when a backend implementation was available, we would have caught this, fixed it in the guests, and not be in the situation because there wouldn't be these broken guests.

So we *still* see the bug re-surface in new guests.

Which guests? Newer versions of dhclient should work as expected.

And since they are fairly new, it is interesting to
get decent performance from them now.

I don't think it makes sense for vhost to do this. These guests are
so old that they don't have the requisite features to achieve really
high performance anyway.

I've always thought making vhost totally transparent was a bad idea
and this is one of the reasons.
It does not have to be fully transparent. You can insert your own ring
in the middle, and copy descriptors around. And we stop on errors and
let userspace handle. This will come handy if we get e.g. virtio bug
that we need to work around.

I mean from a UI perspective. IOW, if users have to explicitly choose to use vhost-net, then it's okay to force them to use newer guests that support vhost-net. However, if we make it transparent, then it has to support everything that QEMU virtio has ever supported which is problematic for exactly the reasons you are now encountering.

We can do a lot of ugly things in
userspace that we shouldn't be doing in the kernel.


Anthony Liguori
QEMU is only userspace for the host. It is the hardware for the guest.
So IMO we should not be doing the ugly things there either.

Shouldn't we apply the same argument to the Windows RTC implementation and say that Windows should not rely on counting interrupts? Or that it shouldn't spin in a tight loop checking interrupt status with interrupts disabled after receiving an interrupt?

Supporting broken guests is a big part of what we do in QEMU. We do what we need to do to make guests that we cannot change work. When this first was implemented, there were a good number of pre-existing guests that broke because we enabled checksum offload.

If we can fix the guests to avoid doing ugly things in QEMU, we should, but we can't regress an otherwise working guest just because we think the solution is ugly in QEMU.


Anthony Liguori

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