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Re: [libvirt] support for configuring all ports of a multiport SRIOV VF when assigning to guest



On 08/03/2017 02:33 AM, Moshe Levi wrote:
> Hi Laine,
>
> I have a few question before I can give my opinion.
> I the Mellanox Card Dual Port that support one PCI with 2 PF is  ConnectX-3 and ConnectX-3 Pro. (maybe others cards  I will check this)
> The ConnectX-4 Dual Port and above is implemented with 2 PCI devices per 2 PF.

So is the "multiple netdevs for a single PCI device" hardware model
completely deprecated, and will never show up again in new products?

If that's the case, maybe I shouldn't burden libvirt's config with all
this new config that will only be used for legacy hardware. Perhaps a
better approach would be to stick with the current config, and make it
work properly for VFIO device assignment when a ConnectX-3 card is
configured in single port mode - even that doesn't work correctly now[*]
but it's a more easily solved problem, and can be done with no config
changes.

Opinions about this? If it's a deadend and the existing legacy hardware
can be used in a reasonable manner by setting it to single port mode, I
don't want to add externally-visible knobs to libvirt.


[*]If a VF is configured to be "port 2 only", libvirt would still try to
get/set the MAC and vlan tag with a netlink message to the *port 1*
netdev of the PF, so it would be saving/setting the wrong (nonexistent?)
VF netdev.

I did send patches yesterday (any reviews/testing appreciated!) that
make everything work properly when saving/setting/restoring the VF
netdev MAC/vlan tags on dual port cards used for macvtap passthrough:

  https://www.redhat.com/archives/libvir-list/2017-August/msg00170.html

Those don't fix the situation when doing VFIO device assignment, but
they do at least make macvtap passthrough work correctly (for all VF
netdevs, even when the netdevs are dual port!), and are the first step
in getting it right for VFIO.


> I can check with our driver architect why it was done like this in the past.

They will obviously have much better insight than me :-), but my
understanding from the outside is that one reason for doing it this way
was to lessen the total amount of MMIO space usage on the host. Since
each VF (PCI device) uses 8MB or something of MMIO, which can really add
up when you're talking about the difference between e.g. 64 VFs for 128
netdevs in dual port mode vs 128 VFs for 128 netdevs in single port mode
(but of course this assumes that it's okay/desirable to assign the
netdevs to guests in pairs.


> The pci address in the xml below is VF pci which is different between all VF so I am not sure why it causing problems with libvirt for setting mac? 

The problem is that in dual port mode, each VF has *2* netdevs
associated with it. Each of those netdevs has its own MAC address, but
is a part of the same PCI device. It would of course be possible to just
add another MAC address to the <interface type='hostdev'> element, but I
dislike that because I think of <interface> as being "a single network
device" and adding config for a 2nd network device is breaking that
model. (i.e. it would work, but it looks ugly, and gets uglier when you
add vlan tag to the mix)


> <source>
>              <address type='pci' slot='0x08' function='0x4'/>
> </source>
>
>
> I will do a little shift to openstack with SR-IOV mechanism driver.
> I remember with try to enable support on the second port for such  cards in openstack 
> I remember we tested Mellanox ConnectX-3  Pro Dual Port  with openstack to allow boot a vm on both ports.
> I implemented the pci-passthrough-whitelist-regex  to allow a flexibly way to whitelist pci device.
> And we also had a patch in neutron for the SR-IOV to allow the agent to allow mapping of multiple PFs to a PCI device, but the community didn't like it especially intel. 

Looking through that, it sounds like the person from Intel had never
before heard of a card that put separate netdevs on the same PCI device,
and he didn't really understand the concept. He was trying to see
everything in the "1 netdev for 1 PCI address" model that he was used
to, and in that framework what you were saying didn't make sense to him.

(for my part, I don't understand openstack code, so I don't really
understand the details of what your patch was trying to do, but that's a
separate / orthogonal problem; I *do* understand why it would need to be
handled differently, at least :-).


I guess mis-connects like this may be part of the reason new Mellanox
cards have shifted away from the "multiple netdevs on a single PCI
address" model - it's difficult to explain to non-involved parties, and
there is a *ton* of code written to assume a 1:1 correspondence that all
breaks when you try to make it 2:1.


> [1] - https://specs.openstack.org/openstack/nova-specs/specs/liberty/approved/pci-passthrough-whitelist-regex.html 
> [2] - https://review.openstack.org/#/c/409526/
>
>
>
>
> -----Original Message-----
> From: sendmail [mailto:justsendmailnothingelse gmail com] On Behalf Of Laine Stump
> Sent: Thursday, August 3, 2017 7:09 AM
> To: Libvirt <libvir-list redhat com>
> Cc: Doug Ledford <dledford redhat com>; Moshe Levi <moshele mellanox com>; Daniel P. Berrange <berrange redhat com>
> Subject: RFC: support for configuring all ports of a multiport SRIOV VF when assigning to guest
>
> ("No matter how far you've gone down the wrong road, turn back." - paraphrase of a Turkish proverb that is apropos to this discussion)
>
> Several years ago, when I was apparently naive and narrow in my thinking and someone wanted us support setting the MAC address and vlan tag for SRIOV VFs when assigning them to a guest with PCI device assignment (this was before VFIO existed), I had the idea to do this by creating a new type of <interface> device:
>
>    <interface type='hostdev'>
>     ....
>
> My thinking was that <interface> already had elements for mac address, 802.11Qb[gh] virtualport config, and vlan tag (or maybe it was that we were *going to add* support for vlan tag), so by just adding a <source> that was a PCI address, we would have everything we needed. Basically, there is some amount of config that needs to be applied to the device before it's assigned to the guest, and since the device ends up being a netdev in the guest, all that config is already present in an <interface>. As a bonus, because it was an <interface> we could easily re-use the recently added "pool of devices" network type (with some minor adjustment) to avoid needing to hardcode the host-side PCI address of the VF.
>
> At the time Dan Berrange countered (I think - correct me if I'm wrong!) that we should instead do this with modifications to <hostdev>, but somehow I managed to either convince him, or maybe he just finally tired of my stubbornness and decided it was easier to deal with the after effects of giving in rather than continuing to debate with me :-)
>
> So right now if you want to assign an SRIOV VF network device to a guest with VFIO, you need something like this (ignoring network device pools for the moment):
>
>     <interface type='hostdev'>
>       <source>
>         <address type='pci' slot='0x08' function='0x4'/>
>       </source>
>       <mac address='52:54:00:01:01:01'/>
>       <vlan>
>         <tag id='42'/>
>       </vlan>
>     </interface>
>
> (or in place of <vlan>, you could have a <virtualport> element for 802.11Qb[gh]).
>
>
> The SRIOV cards that we had around when we were doing this work had multiple physical ports on them (either 2 or 4), but each physical port was associated with its own PCI Physical Function (PF), and each of the PCI Virtual Functions associated with a PF was tied to a single netdev, i.e. in all cases there was always a 1:1 correspondence between a netdev and a PCI device. All of libvirt's code dealing with SRIOV VFs and PFs assumes this 1:1 relationship.
>
> And then came Mellanox "dual port" SRIOV cards....
>
> A Mellanox SRIOV NIC doesn't necessarily do that. Instead, it can operate in "dual port" mode, where it has a single PCI PF device for both physical ports; the single PF PCI device has 2 separate netdevs associated with it (so when you look in the "net" subdirectory for the PCI device, you'll see two netdevs listed, and when you look in the "device" subdirectory of those two netdevs in sysfs, they both point back to the same PCI device). VFs associated with that PF will also each have two netdevs associated with them. This means that when you assign a VF to a guest, the guest is getting a single PCI device, but it's getting two netdevs. (I've been told that the advantage of doing both ports with a single PCI device is that each Mellanox PCI device uses a huge amount of MMIO space, two ports on each device cuts the MMIO usage in half).
>
> In order for this to be useful, libvirt needs to set the mac address and vlan tag of *both* netdevs prior to starting the guest. But we have no way to represent that in our configuration. In the past it's been suggested that we just do something like this:
>
>    <interface type='hostdev'>
>      <mac address='blah'/>
>      <mac2 address='blah'/>
>      ...
>    </interface>
>
> but I have two problems with that:
>
> 1) <interface> is supposed to represent a single network device, but this is trying to make it represent 2 network devices (and what if someone else comes up with a card that puts *4* netdevs on the same PCI
> device?)
>
> 2) We would need to do the same thing for <vlan> tag. It starts to get ugly.
>
> Alternately we could add a new <port number='2'> subelement, like this:
>
>
>     <interface type='hostdev'>
>       <source>
>         <address type='pci' slot='0x08' function='0x4'/>
>       </source>
>       <mac address='52:54:00:01:01:01'/>
>       <vlan>
>         <tag id='42'/>
>       </vlan>
>       <port number='2'>
>         <mac address='52:54:00:01:01:01'/>
>         <vlan>
>           <tag id='42'/>
>         </vlan>
>       </port>
>     </interface>
>
> (or some variation of that) just so that all the stuff for the 2nd port is grouped together. But I don't like that the config for port 1 is at a different level in the hierarchy than the config for port 2, and we still have the problem that we're trying to describe *2* netdevs with a single <interface> element, which just feels wrong.
>
> - OR -
>
> what if we admit that <interface type='hostdev'> was a bad idea, and try doing it all with <hostdev>, something like this:
>
>   <hostdev mode='subsystem' type='pci' managed='yes'>
>     <source>
>       <address domain='0x0000' bus='0x06' slot='0x02' function='0x0'/>
>     </source>
>     <netdev port='1'>
>       <mac address='52:54:00:01:02:03'/>
>       <vlan>
>         <tag id='42'/>
>       </vlan>
>     </netdev>
>     <netdev port='2'>
>       <mac address='52:54:00:01:02:03'/>
>       <vlan>
>         <tag id='43'/>
>       </vlan>
>     </netdev>
>   </hostdev>
>
> The downsides are:
>
> 1) It's providing a 2nd way of describing single port VFs, which could confuse people (my recommendation would be to deprecate usage of <interface type='hostdev'> in the documentation, while still allowing it; i.e. we'd still have to maintain that code while discouraging its use).
>
> 2) This wouldn't be able to take advantage of the pools of devices maintained by libvirt networks. (This isn't a problem for Openstack, since they don't use that anyway, but ovirt does use it).
>
> 3) It's an explicit admission that I made a bad decision in 2011 :-P
>
> The upsides?
>
> 1) it models the hardware more correctly. (it really is a PCI device that has two subordinate netdevs, *not* a netdev that is part of a PCI device, "oh and that PCI device also has another netdev")
>
> 2) it could be more logically and easily expanded if there were more ports, or if there were other types of PCI devices that had different kinds of device-type-specific config that needed to be setup.
>
> 3) we could eliminate "downside (2)" by enhancing the nodedevice driver to provide and manage more generalized pools of devices (if desired by anyone - Openstack's opinion seems to be that libvirt shouldn't be doing this anyway).
>
>
> So does anyone have an opinion about this? An alternate proposal? (e.g.
> Should we instead just tell everyone to run their Mellanox cards in single port mode and ignore/avoid all this complexity?)
>


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