From mboxrd@z Thu Jan  1 00:00:00 1970
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH v1 2/2] dma-mapping-common: add DMA attribute -
 DMA_ATTR_IOMMU_BYPASS
Date: Fri, 30 Oct 2015 10:51:54 +0900
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Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
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To: Andy Lutomirski <luto@amacapital.net>
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On Thu, 2015-10-29 at 11:31 -0700, Andy Lutomirski wrote:
> On Oct 28, 2015 6:11 PM, "Benjamin Herrenschmidt"
> <benh@kernel.crashing.org> wrote:
> > 
> > On Thu, 2015-10-29 at 09:42 +0900, David Woodhouse wrote:
> > > On Thu, 2015-10-29 at 09:32 +0900, Benjamin Herrenschmidt wrote:
> > > 
> > > > On Power, I generally have 2 IOMMU windows for a device, one at
> > > > the
> > > > bottom is remapped, and is generally used for 32-bit devices
> > > > and the
> > > > one at the top us setup as a bypass
> > > 
> > > So in the normal case of decent 64-bit devices (and not in a VM),
> > > they'll *already* be using the bypass region and have full access
> > > to
> > > all of memory, all of the time? And you have no protection
> > > against
> > > driver and firmware bugs causing stray DMA?
> > 
> > Correct, we chose to do that for performance reasons.
> 
> Could this be mitigated using pools?  I don't know if the net code
> would play along easily.

Possibly, the pools we have already limit the lock contention but we
still have the map/unmap overhead which under a hypervisor can be quite
high. I'm not necessarily against changing the way we do things but it
would have to be backed up with numbers.

Cheers,
Ben.