Re: [PATCH 0/9][RFC] KVM virtio_net performance

Re: [PATCH 0/9][RFC] KVM virtio_net performance

[Rusty Russell]

On Friday 25 July 2008 06:56:01 you wrote:
> I'm still seeing the same problem I saw with my patch series.  Namely,
> dhclient fails to get a DHCP address.

This is, from one point of view, a dhclient bug.  It opens a packet socket, 
and we'd tell it that the checksum is incomplete via auxdata, but it doesn't 
have auxdata.

On the other hand, perhaps we should copy the packet and checksum it in this 
case?  Not many devices input packets with partial csums, so dhclient doesn't 
normally hit this.  Xen might though?

ie, in net/packet/af_packet.c's packet_recvmsg():

	if (pkt_sk(sk)->auxdata) {
		struct tpacket_auxdata aux;

		aux.tp_status = TP_STATUS_USER;
		if (skb->ip_summed == CHECKSUM_PARTIAL)
			aux.tp_status |= TP_STATUS_CSUMNOTREADY;
		aux.tp_len = PACKET_SKB_CB(skb)->origlen;
		aux.tp_snaplen = skb->len;
		aux.tp_mac = 0;
		aux.tp_net = skb_network_offset(skb);
		aux.tp_vlan_tci = skb->vlan_tci;

		put_cmsg(msg, SOL_PACKET, PACKET_AUXDATA, sizeof(aux), &aux);
	}

add an "else if (skb->ip_summed == CHECKSUM_PARTIAL)" case...

Rusty.

Re: [PATCH 0/9][RFC] KVM virtio_net performance

[Herbert Xu]

Rusty Russell <rusty@rustcorp.com.au> wrote:
>
> add an "else if (skb->ip_summed == CHECKSUM_PARTIAL)" case...

If you're going to hack the guest kernel, then you might as well
fix the guest DHCP client.  However, we should default checksum
offload to off until we know that the client is capable of handling
it (to handle guests that haven't been fixed).

One easy way of doing this is to hook up the rx checksum offload
option in the guest with the tx offload option in the host.  In
other words, when rx offload is enabled in the guest we enable
tx offload in the host, and disable it vice versa.

Cheers,
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csum offload and af_packet

[Rusty Russell]

On Monday 11 August 2008 19:51:39 Herbert Xu wrote:
> Rusty Russell <rusty@rustcorp.com.au> wrote:
> > add an "else if (skb->ip_summed == CHECKSUM_PARTIAL)" case...
>
> If you're going to hack the guest kernel, then you might as well
> fix the guest DHCP client.  However, we should default checksum
> offload to off until we know that the client is capable of handling
> it (to handle guests that haven't been fixed).

I think this is deeper than that.  This case is actually unusual, in that the 
packet really does arrive with a partial csum.  But usually, we're exposing 
an internal detail of our stack at this point.  Seems like we shouldn't if we 
know the user can't deal with it.  dhcpd just makes this case less academic.

> One easy way of doing this is to hook up the rx checksum offload
> option in the guest with the tx offload option in the host.  In
> other words, when rx offload is enabled in the guest we enable
> tx offload in the host, and disable it vice versa.

We can trivially disable it in the guest or host; that's not the problem.  We 
can even disable csum offload just for UDP in the host.  But should we 
really?

Rusty.

Re: csum offload and af_packet

[Herbert Xu]

On Mon, Aug 11, 2008 at 11:50:25PM +1000, Rusty Russell wrote:
>
> I think this is deeper than that.  This case is actually unusual, in that the 
> packet really does arrive with a partial csum.  But usually, we're exposing 
> an internal detail of our stack at this point.  Seems like we shouldn't if we 
> know the user can't deal with it.  dhcpd just makes this case less academic.

I disagree.  If you're using AF_PACKET you're asking to see the
bare details.  If you want to see the censored version you can
always go through the IP stack.

> We can trivially disable it in the guest or host; that's not the problem.  We 
> can even disable csum offload just for UDP in the host.  But should we 
> really?

It's not about disabling it, it's about enabling it dynamically
once guest user-space is sure that *it* can handle this.

Cheers,`
-- 
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Re: csum offload and af_packet

[David Miller]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Tue, 12 Aug 2008 08:32:44 +0800

> On Mon, Aug 11, 2008 at 11:50:25PM +1000, Rusty Russell wrote:
> >
> > I think this is deeper than that.  This case is actually unusual, in that the 
> > packet really does arrive with a partial csum.  But usually, we're exposing 
> > an internal detail of our stack at this point.  Seems like we shouldn't if we 
> > know the user can't deal with it.  dhcpd just makes this case less academic.
> 
> I disagree.  If you're using AF_PACKET you're asking to see the
> bare details.  If you want to see the censored version you can
> always go through the IP stack.

Unfortunately the censored version doesn't allow you to get at the
link level headers, which is what at least some of these applications
want.

Re: csum offload and af_packet

[Herbert Xu]

On Mon, Aug 11, 2008 at 05:51:54PM -0700, David Miller wrote:
>
> Unfortunately the censored version doesn't allow you to get at the
> link level headers, which is what at least some of these applications
> want.

Yes that is certainly true for the DHCP server, but Rusty was
complaining about the DHCP client which certainly does not need
the LL headers on reception (although using it is definitely more
convenient since you need it on transmit anyway).

I agree that handling this in AF_PACKET is certainly possible,
and for that matter not extremely difficult.  However, my point is
that doing this for the purposes of virtualisation is completely
pointless.

The only time you need this is when you have an old guest that
you cannot modify.  If you could modify it you can always give it
a patched DHCP client (I've alread patched dhcp-client for Xen
so that should work properly for KVM/lguest if you're using the
latest version).

And if you can't modify it then patching AF_PACKET is completely
pointless since we need to do this in the guest kernel and not the
host kernel.

Cheers,
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Re: csum offload and af_packet

[Rusty Russell]

On Tuesday 12 August 2008 10:32:44 Herbert Xu wrote:
> On Mon, Aug 11, 2008 at 11:50:25PM +1000, Rusty Russell wrote:
> > I think this is deeper than that.  This case is actually unusual, in that
> > the packet really does arrive with a partial csum.  But usually, we're
> > exposing an internal detail of our stack at this point.  Seems like we
> > shouldn't if we know the user can't deal with it.  dhcpd just makes this
> > case less academic.
>
> I disagree.  If you're using AF_PACKET you're asking to see the
> bare details.  If you want to see the censored version you can
> always go through the IP stack.

Then should we insist the user set PACKET_AUXDATA?  Even then, the format of 
that cmsg will have to be enhanced as we change kernel internals.  Which is 
probably why you *don't* get to see the bare details: you get a flag 
saying "oh, I know the checksum is bad".  Without the csum_start/csum_offset 
fields you can't even calculate what it will be.

The dhcp client thing is a symptom which can be fixed, but are we doing the 
right thing?  (Tho for lguest this is a new problem with the current kernel, 
so fixing it now means it really wouldn't be a problem).

> > We can trivially disable it in the guest or host; that's not the problem.
> >  We can even disable csum offload just for UDP in the host.  But should
> > we really?
>
> It's not about disabling it, it's about enabling it dynamically
> once guest user-space is sure that *it* can handle this.

Oh, I see.  I'd have to think harder; I'm not sure if we have all the pieces 
at the moment for virtio or would need a boutique mechanism for this (usually 
the host doesn't change the features it offers, even if device resets).  May 
be easier to suppress csum offload for dhcp packets in the host...

Rusty.

Re: csum offload and af_packet

[Ingo Oeser]

Herbert Xu schrieb:
> The only time you need this is when you have an old guest that
> you cannot modify.  If you could modify it you can always give it
> a patched DHCP client (I've alread patched dhcp-client for Xen
> so that should work properly for KVM/lguest if you're using the
> latest version).

Are you talking about modifying the KVM client image?
There may be reasons, why this is impossible or at least highly undesired.

Before virtualisation developers of embedded stuff need to seal away
their whole machines for their development and test environment 
together with sample hardware.

Now using virtualisation they can at least virtualize their development
and test environment, save lots of cost and not risking that their sealed 
away hardware will not start again, when they need to fix a critical bug 
10-20 years later.

Now try to have them change the KVM client image :-)

Extreme special case, not worth optimizing for, I know. But that special
case is a legal one like any other special case we care about now.


Best Regards

Ingo Oeser

Re: csum offload and af_packet

[Herbert Xu]

On Tue, Aug 12, 2008 at 06:17:19PM +0200, Ingo Oeser wrote:
>
> Are you talking about modifying the KVM client image?
> There may be reasons, why this is impossible or at least highly undesired.
> 
> Before virtualisation developers of embedded stuff need to seal away
> their whole machines for their development and test environment 
> together with sample hardware.
> 
> Now using virtualisation they can at least virtualize their development
> and test environment, save lots of cost and not risking that their sealed 
> away hardware will not start again, when they need to fix a critical bug 
> 10-20 years later.

Oh I totally agree that there are lots of scenarios where you
want to have an unmolested guest image.  My point was that if
you're going to touch the guest kernel anyway you might as well
fix the guest user-space instead.

This is also why I've argued that the default should be to disable
TX checksums until the guest enables it so that old guests that
know nothing about this can continue to work.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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Re: csum offload and af_packet

[David Miller]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Wed, 13 Aug 2008 09:37:53 +1000

> Oh I totally agree that there are lots of scenarios where you
> want to have an unmolested guest image.  My point was that if
> you're going to touch the guest kernel anyway you might as well
> fix the guest user-space instead.
> 
> This is also why I've argued that the default should be to disable
> TX checksums until the guest enables it so that old guests that
> know nothing about this can continue to work.

However, I think Rusty may have a point.

The whole point of the auxdata thing was that if your tool is smart
you can get the notification that the checksum is going to be HW
computed and therefore keep capturing at a very low cost.

But for a stupid AF_PACKET user, like dhclient currently is, there is
no harm in COW'ing the packet header area and calculating the checksum
for them in this case.

I kind of hope that libpcap/tcpdump has been taught about auxdata
by now, has it? :-/

Re: csum offload and af_packet

[Herbert Xu]

On Tue, Aug 12, 2008 at 05:55:32PM -0700, David Miller wrote:
> 
> The whole point of the auxdata thing was that if your tool is smart
> you can get the notification that the checksum is going to be HW
> computed and therefore keep capturing at a very low cost.
> 
> But for a stupid AF_PACKET user, like dhclient currently is, there is
> no harm in COW'ing the packet header area and calculating the checksum
> for them in this case.

Right I'm not against adding this to AF_PACKET.

It's just that the virtualisation problem that started all this
is just as easily resolved by user-space.  In fact the dhcp-client
code in RHEL/Fedora has been patched to do exactly this a long time
ago.

> I kind of hope that libpcap/tcpdump has been taught about auxdata
> by now, has it? :-/

Unfortunately no.  This also means that if we do do this in AF_PACKET,
then running a packet dump on a fast outbound interface may become more
expensive.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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Re: csum offload and af_packet

[David Miller]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Wed, 13 Aug 2008 11:09:35 +1000

> On Tue, Aug 12, 2008 at 05:55:32PM -0700, David Miller wrote:
> > I kind of hope that libpcap/tcpdump has been taught about auxdata
> > by now, has it? :-/
> 
> Unfortunately no.  This also means that if we do do this in AF_PACKET,
> then running a packet dump on a fast outbound interface may become more
> expensive.

It seems pointless for us to add features like mmap() support and
auxdata if the most important client library and tool users of these
features never take it up.

libpcap and tcpdump have been notoriously difficult in this area.

Re: csum offload and af_packet

[Herbert Xu]

On Tue, Aug 12, 2008 at 06:17:31PM -0700, David Miller wrote:
>
> It seems pointless for us to add features like mmap() support and
> auxdata if the most important client library and tool users of these
> features never take it up.

Well auxdata at least is used by dhcp-common and therefore dhcp-client
and dhcp-server.

> libpcap and tcpdump have been notoriously difficult in this area.

Yes the disconnect between the kernel community and the rest is
large.  The only successful examples seems to be where the user-
space code is maintained by a kernel developer, e.g., iproute.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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Re: csum offload and af_packet

[David Miller]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Wed, 13 Aug 2008 11:21:29 +1000

> On Tue, Aug 12, 2008 at 06:17:31PM -0700, David Miller wrote:
> >
> > It seems pointless for us to add features like mmap() support and
> > auxdata if the most important client library and tool users of these
> > features never take it up.
> 
> Well auxdata at least is used by dhcp-common and therefore dhcp-client
> and dhcp-server.

Only because _YOU_ added it :-)

> > libpcap and tcpdump have been notoriously difficult in this area.
> 
> Yes the disconnect between the kernel community and the rest is
> large.  The only successful examples seems to be where the user-
> space code is maintained by a kernel developer, e.g., iproute.

Isn't that the truth...

Alexey was maintaining his own semi-fork of libpcap and tcpdump for
years for the same reasons.

Let's write a real packet capturing tool that will be fast and that we
can add quick support for new kernel features to.

I mean, what other alternative is there?  The pcap/tcpdump folks have
had 10+ years to get their act in gear.

Re: csum offload and af_packet

[Herbert Xu]

On Tue, Aug 12, 2008 at 06:25:06PM -0700, David Miller wrote:
> 
> Alexey was maintaining his own semi-fork of libpcap and tcpdump for
> years for the same reasons.

He had a whole suite of things, some of which are now finally
making it into the mainstream distros.

> Let's write a real packet capturing tool that will be fast and that we
> can add quick support for new kernel features to.
>
> I mean, what other alternative is there?  The pcap/tcpdump folks have
> had 10+ years to get their act in gear.

Maybe we could start from Alexey's code base.

Cheers,
-- 
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Re: csum offload and af_packet

[Patrick McHardy]

Herbert Xu wrote:
> On Tue, Aug 12, 2008 at 05:55:32PM -0700, David Miller wrote:
>
>   
>> I kind of hope that libpcap/tcpdump has been taught about auxdata
>> by now, has it? :-/
>>     
>
> Unfortunately no.  This also means that if we do do this in AF_PACKET,
> then running a packet dump on a fast outbound interface may become more
> expensive.

Actually the current CVS version understands auxdata and uses it to
reconstruct VLAN headers. CVS seems to be broken though, I can send
you a tarball with the current version if you want.

Re: csum offload and af_packet

[David Miller]

From: Patrick McHardy <kaber@trash.net>
Date: Wed, 13 Aug 2008 13:26:22 +0200

> Actually the current CVS version understands auxdata and uses it to
> reconstruct VLAN headers. CVS seems to be broken though, I can send
> you a tarball with the current version if you want.

At first I thought you were saying that the code in their current
CVS is broken, but now I understand that CVS itself is broken for
this project.

That's rediculious.

I'll see if I can put some time together to try and put some trees
together, please send me what you have Patrick, thanks.

If I'm successful I'll put GIT trees up on kernel.org for tcpdump
and libpcap which we can hack on.

Thanks guys.

Re: csum offload and af_packet

[Herbert Xu]

On Sun, Aug 17, 2008 at 04:08:25PM -0700, David Miller wrote:
>
> I'll see if I can put some time together to try and put some trees
> together, please send me what you have Patrick, thanks.
> 
> If I'm successful I'll put GIT trees up on kernel.org for tcpdump
> and libpcap which we can hack on.

That's awesome! Thanks Dave!
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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Re: csum offload and af_packet

[David Miller]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Mon, 18 Aug 2008 11:10:18 +1000

> On Sun, Aug 17, 2008 at 04:08:25PM -0700, David Miller wrote:
> >
> > I'll see if I can put some time together to try and put some trees
> > together, please send me what you have Patrick, thanks.
> > 
> > If I'm successful I'll put GIT trees up on kernel.org for tcpdump
> > and libpcap which we can hack on.
> 
> That's awesome! Thanks Dave!

There is a libpcap.git up in my dir on kernel.org:

	master.kernel.org:/pub/scm/linux/kernel/git/davem/libpcap.git

It's just a bare 0.9.8 import plus a commit to get rid of all of the
rcsid strings.

Re: csum offload and af_packet

[David Miller]

From: Patrick McHardy <kaber@trash.net>
Date: Mon, 18 Aug 2008 03:13:35 +0200

> Actually both I guess, the current version finally has mmaped
> packet socket support and the linux support looks a lot cleaner
> in general, but I stumbled over a few bugs that indicate
> alpha-state. The maintainer is very responsive though.
 ...
> I've attached the last snapshot I have.

Thanks Patrick.

Re: csum offload and af_packet

[David Miller]

From: Patrick McHardy <kaber@trash.net>
Date: Mon, 18 Aug 2008 03:13:35 +0200

> the linux support looks a lot cleaner in general

I totally disagree after having looked at this thing.

The number of ifdefs has doubled in pcap-linux.c since the last
official release, it's quite a mess.  It's still handling all sorts of
obscure bugs we had back in 2.0.x as well.

Re: csum offload and af_packet

[Patrick McHardy]

David Miller wrote:
> From: Patrick McHardy <kaber@trash.net>
> Date: Mon, 18 Aug 2008 03:13:35 +0200
> 
>> the linux support looks a lot cleaner in general
> 
> I totally disagree after having looked at this thing.

You should have seen the old mmaped packet socket support :)
But I agree, it certainly could be done cleaner.