[openib-general] FMR and how they work

[Dror Goldenberg]

> -----Original Message-----
> From: Fab Tillier [mailto:ftillier at infiniconsys.com] 
> Sent: Thursday, April 28, 2005 6:00 PM
> 
> 
> When you deregister a FMR, what information from that FMR can 
> the HCA cache
> - just the MTTs or also the MPTs?  

Both. I assume that by "deregister FMR" you refer to ib_fmr_pool_unmap(),
which only takes the FMR and puts it back into a pool.
If you fully destroy an FMR object then the mapping is disabled.

> Can an incoming RDMA 
> transfer access the pages previously referenced by that FMR?  
> If so, for how long?  When is it safe to unpin the pages?

As long as the entries are cached, then the answer is yes. Obviously
according to the FMR access rights and PD which are always validated. 
The entries get out of the cache either when cache is trashed or when 
you explicitly flush it. If you want explicit cache flush, then you can 
use SYNC_TPT  command.
Destruction of the object (HW2SW_MPT) also updates the caches
but is an expensive operation.

> 
> What about when you modify the FMR?  How long are previous 
> mappings "at risk" of transfers?

Same answer.

> 
> I'm kinda confused because the Mellanox docs indicate that 
> the HCA can cache stuff and that the pages can't be unpinned 
> until the cache is flushed.  If that's true, I don't see how 
> FMRs can be used because they could be suspect to malicious 
> use by a remote host.

It's a tradeoff. If you want bulletproof against malicious user then you'd
need to invalidate the FMR by destroying the object in each IO operation. 
If you want high performance, then you need to trust your peer to some 
extent.
Looking into the future, then Extended verbs will enable a light operation 
to invalidate such objects, which will facilitate improvements of security.

> 
> Help?
> 
> - Fab
> 
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