[openib-general] mthca FMR correctness (and memory windows)

[Dror Goldenberg]

> From: Talpey, Thomas [mailto:Thomas.Talpey at netapp.com] 
> Sent: Monday, March 20, 2006 10:20 PM
> 
> At 02:58 PM 3/20/2006, Roland Dreier wrote:
> >If you want to invalidate each FMR individually, then  there's not
much 
> >point in using FMRs at all.  Just register and unregister memory 
> >regions.

I'd say +/- here, because map() speeds up the registration (no
need for cmdif), and unmpap() may slow down the deregistration.

> 
> Really? The idea of FMR, I thought, was to preallocate the 
> TPT entry up front (alloc_fmr), and then populate it with the 
> lightweight fmr api (map_phys_fmr).
> 
> If I use ib_reg_phys_mr(), then I incur all the preallocation 
> overhead each time. Sure the benefit of an FMR isn't merely 
> that the hard work is deferred, opening a vulnerability while 
> it's pending??
> 

I'd rather call this a tradeoff between performance and exposure.
You probably also trust your NFS server not to return you garbage 
or malicious executable files when you're reading from the network
filesystem. You might as well just trust it not to do other malicious 
stuff.
I'd leave it for you to decide, but I strongly recommend on trying the
API even just for the sake of performance assessment.