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Date:	Fri, 8 May 2015 00:23:47 +0000
From:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
To:	"Skidmore, Donald C" <donald.c.skidmore@...el.com>,
	Or Gerlitz <gerlitz.or@...il.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
CC:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"nhorman@...hat.com" <nhorman@...hat.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>,
	"jogreene@...hat.com" <jogreene@...hat.com>,
	"Choi, Sy Jong" <sy.jong.choi@...el.com>,
	Edward Cree <ecree@...arflare.com>,
	Rony Efraim <ronye@...lanox.com>
Subject: RE: [net-next 07/11] if_link: Add VF multicast promiscuous control

> > -----Original Message-----
> > From: Hiroshi Shimamoto [mailto:h-shimamoto@...jp.nec.com]
> > Sent: Wednesday, May 06, 2015 10:55 PM
> > To: Skidmore, Donald C; Or Gerlitz; Kirsher, Jeffrey T
> > Cc: David Miller; Linux Netdev List; nhorman@...hat.com;
> > sassmann@...hat.com; jogreene@...hat.com; Choi, Sy Jong; Edward Cree;
> > Rony Efraim
> > Subject: RE: [net-next 07/11] if_link: Add VF multicast promiscuous control
> >
> > > > -----Original Message-----
> > > > From: Or Gerlitz [mailto:gerlitz.or@...il.com]
> > > > Sent: Sunday, May 03, 2015 7:16 AM
> > > > To: Kirsher, Jeffrey T
> > > > Cc: David Miller; Hiroshi Shimamoto; Linux Netdev List;
> > > > nhorman@...hat.com; sassmann@...hat.com; jogreene@...hat.com;
> > Choi,
> > > > Sy Jong; Edward Cree; Skidmore, Donald C; Rony Efraim
> > > > Subject: Re: [net-next 07/11] if_link: Add VF multicast promiscuous
> > > > control
> > > >
> > > > On Sat, May 2, 2015 at 1:42 PM, Jeff Kirsher
> > > > <jeffrey.t.kirsher@...el.com>
> > > > wrote:
> > > > > From: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
> > > > >
> > > > > Add netlink directives and ndo entry to allow VF multicast
> > > > > promiscuous
> > > > mode.
> > > > >
> > > > > This controls the permission to enter VF multicast promiscuous mode.
> > > > > The administrator will dedicatedly allow multicast promiscuous per VF.
> > > > >
> > > > > When the VF is under multicast promiscuous mode, all multicast
> > > > > packets are sent to the VF.
> > > > >
> > > > > Don't allow VF multicast promiscuous if the VM isn't fully trusted.
> > > >
> > > > Guys,
> > > >
> > > > I don't think the discussion we held in the past [1] on the matter
> > > > actually converged. Few open points that came up while debating it
> > > > internally with
> > > > Rony:
> > > >
> > > > 1. maybe what we we actually want here an API that states a VF to be
> > > > privileged/trusted and then we can over load the feature set of being
> > such?
> > >
> > > I suggested this originally, but there was push back as it was thought too
> > generic as the definition of what being a "trusted"
> > > vendor would differ from driver to driver.  Personally I still like the idea of
> > having one mode saying that we "trust"
> > > a given VF.  Then that VF can request whatever it support it wants
> > > from the PF regardless of possible negative impact on other VF's.
> > > What is possible to support would then be left to the interface
> > > between the VF and PF.  This of course would be dependent on what the
> > given HW could support and would mean this mode would mean different
> > things for different adapters and I do see why some might see this as a
> > concern.
> >
> > The point is granularity, right?
> > Allow everything or allow subset of features.
> 
> Nice way to sum it up.  The trick being with the subset of features path is not all hardware can/will support everything.
> Also I worry about worry about the feature list growing requiring more and more nobs on the PF to allow/disallow granular
> behavior that could brake VF isolation.  With a simple hint to the PF that a given VF is "trusted" would allow all that
> complexity to be contained in the mailbox protocol between the PF/VF.
> 
> All that said I realize others are concerned with the ambiguousness of such a field and can certainly live with your
> implementation.

I see, it seems better to have a single knob which indicates "trust this VF" and PF will allow requests which
might hurt performance or security from trusted VF, instead of creating a knob for multicast promiscuous,
a knob for feature X and so on.

I will make a patch to implement that "trusted knob" instead of allowing MC promiscuous.
Is there any comment?

> 
> >
> > >
> > > >
> > > > 2. the suggested API only allows either unlimited number of mulicast
> > > > groups per VF or limited number, both numbers are vendor dependent,
> > right?
> > > > maybe what we need for this specific matter is specifying how many
> > > > multicast groups are allowed for a VF?
> > >
> > > I believe the idea behind this interface was that it would allow VF's
> > > to request unlimited multicast group as opposed to the current
> > > behavior of each adapter offering some limited number.  This limit is
> > > of course defined by a given adapters HW/SW limitations.  Up until now
> > > you could keep asking for new multicast until the PF replied with an
> > > error.  So we never really exported this information before.  This new
> > mode just allows us to never reach the point that the PF would deny a VF
> > request to join a MC group.  Seems to me that an additional interface to
> > provide the max number of supported multicast groups would be new
> > functionality that could be independent of this patch and in fact could exist
> > even without this patch.
> > > Or am I missing what you're asking for here. :)
> >
> > I think that the current limitation of multicast on ixgbevf comes from the
> > implementation of mailbox API between VF and PF which has 32 words.
> >
> 
> In the end the limit on number of MC groups, if you don't use promiscuous mode, is the size of the multicast table array.
> We could be sharing this table better between all users rather than the arbitrary limit, but you would hit a hard limit
> due to the size of the table.

Just to clarify the current implementation, I think there is no hard limit of MC address.
The ixgbe driver uses the MTA (multicast table array) and the MTA is shared among all VFs. VF requests
to register 30 multicast address hash values at most and PF will set the corresponding bit in the MTA.
When multicast packet comes in, NIC checks the MTA bit and transmit the packet every VF. 82599 uses the
12 bits hash value of MC address and there is the MTA which is 4096 bits array, the MTA covers all MC
address to filter. I think if all bits on in the MTA, it means that no MC packet is dropped.

thanks,
Hiroshi

> 
> > By the way, our requirement is to make VF promiscuous mode for SDN/NFV
> > usage.
> > And there is a feature to enable in HW, we'd like to use it.
> > I know there is a possibility of performance degradation.
> >
> > thanks,
> > Hiroshi
> 
> I think your method is the way to go, in that if you ask for more than we allow per VF and the PF has this ability enabled
> we just put the VF into multicast promiscuous mode.  However I don't see the advantage of having an interface to tell
> how many groups need to be requested before this happens.  If you were worried about the performance degradation of entering
> promiscuous multicast don't allow it in the PF, which of course will be the default.
> 
> Thanks,
> -Don Skidmore <donald.c.skidmore@...el.com>
> 
> 

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