linux-kernel - Re: [PATCH 2/2] IB/mad: Use IDR for agent IDs

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Date:   Tue, 12 Jun 2018 08:33:59 -0600
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     jackm <jackm@....mellanox.co.il>
Cc:     Leon Romanovsky <leon@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        hans.westgaard.ry@...cle.com, Doug Ledford <dledford@...hat.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-rdma@...r.kernel.org,
        Håkon Bugge <haakon.bugge@...cle.com>,
        Parav Pandit <parav@...lanox.com>,
        Pravin Shedge <pravin.shedge4linux@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] IB/mad: Use IDR for agent IDs

On Tue, Jun 12, 2018 at 07:59:42AM +0300, jackm wrote:
> On Mon, 11 Jun 2018 10:19:18 -0600
> Jason Gunthorpe <jgg@...lanox.com> wrote:
> 
> > On Mon, Jun 11, 2018 at 09:19:14AM +0300, jackm wrote:
> > > On Sun, 10 Jun 2018 22:42:03 -0600
> > > Jason Gunthorpe <jgg@...lanox.com> wrote:
> > >   
> > > > Er, the spec has nothing to do with this. In Linux the TID is made
> > > > unique because the core code provides 32 bits that are unique and
> > > > the user provides another 32 bits that are unique. The driver
> > > > cannot change any of those bits without risking non-uniquenes,
> > > > which is exactly the bug mlx4 created when it stepped outside its
> > > > bounds and improperly overrode bits in the TID for its own
> > > > internal use.  
> > > 
> > > Actually, the opposite is true here.  When SRIOV is active, each VM
> > > generates its *own* TIDs -- with 32 bits of agent number and 32 bits
> > > of counter.  
> > 
> > And it does it while re-using the LRH of the host, so all VMs and the
> > host are now forced to share a TID space, yes I know.
> > 
> > > There is a chance that two different VMs can generate the same TID!
> > > Encoding the slave (VM) number in the packet actually guarantees
> > > uniqueness here.  
> > 
> > Virtualizing the TID in the driver would be fine, but it must
> > virtualize all the TIDs (even those generated by the HOST).
> 
> It DOES do so.  The host slave id is 0. Slave numbers start with 1.
> If the MS byte contains a zero after paravirtualization, the MAD
> was sent by the host.
> In fact, ALL mads are paravirtualized -- including those to/from the host.

Just assuming the byte is 0 and replacing it with something else is
*NOT* virtualization.

Jason