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

From: Michael S. Tsirkin
Date: Wed Jun 30 2010 - 18:38:20 EST

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

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.

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

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