Re: [PATCH v1 2/2] dma-mapping-common: add DMA attribute - DMA_ATTR_IOMMU_BYPASS

From: David Woodhouse <dwmw2@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Shamir Rabinovitch <shamir.rabinovitch@oracle.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	Sebastian Ott <sebott@linux.vnet.ibm.com>,
	linux-s390 <linux-s390@vger.kernel.org>,
	Cornelia Huck <cornelia.huck@de.ibm.com>,
	Joerg Roedel <jroedel@suse.de>, Jonathan Corbet <corbet@lwn.net>,
	KVM <kvm@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v1 2/2] dma-mapping-common: add DMA attribute - DMA_ATTR_IOMMU_BYPASS
Date: Thu, 29 Oct 2015 22:35:25 +0000	[thread overview]
Message-ID: <1446158125.4471.5.camel@infradead.org> (raw)
In-Reply-To: <CALCETrXrzq1f9sMAbKcmhpEtiyq5FE871=4+YspA=X+RjD7p8Q@mail.gmail.com>

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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.

For the receive side, it shouldn't be beyond the wit of man to
introduce an API which allocates *and* DMA-maps a skb. Pass it to
netif_rx() still mapped, with a destructor that just shoves it back in
a pool for re-use.

Doing it for transmit might be a little more complex, but perhaps still
possible.

-- 
dwmw2

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