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: Tue, 03 Nov 2015 07:13:28 +1100
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Cc: David Woodhouse <dwmw2@infradead.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.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>
To: Shamir Rabinovitch <shamir.rabinovitch@oracle.com>
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On Mon, 2015-11-02 at 14:07 +0200, Shamir Rabinovitch wrote:
> On Mon, Nov 02, 2015 at 09:00:34PM +1100, Benjamin Herrenschmidt
> wrote:
> > 
> > Chosing on a per-mapping basis *in the back end* might still make
> > some
> 
> In my case, choosing mapping based on the hardware that will use this
> mappings makes more sense. Most hardware are not that performance 
> sensitive as the Infiniband hardware.

 ...

> The driver know for what hardware it is mapping the memory so it know 
> if the memory will be used by performance sensitive hardware or not.

Then I would argue for naming this differently. Make it an optional
hint "DMA_ATTR_HIGH_PERF" or something like that. Whether this is
achieved via using a bypass or other means in the backend not the
business of the driver.

> In your case, what will give the better performance - 1:1 mapping or
> IOMMU
> mapping? When you say 'relaxing the protection' you refer to 1:1
> mapping?

> Also, how this 1:1 window address the security concerns that other
> raised
> by other here?

It will partially only but it's just an example of another way the
bakcend could provide some improved performances without a bypass.

Cheers,
Ben.