linux-kernel - Re: extra large DMA buffer for PCI-E device under UIO

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Message-ID: <20111122184040.GB5235@redhat.com>
Date:	Tue, 22 Nov 2011 20:40:40 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Hans J. Koch" <hjk@...sjkoch.de>
Cc:	Jean-Francois Dagenais <jeff.dagenais@...il.com>,
	Greg KH <gregkh@...e.de>, tglx@...utronix.de,
	linux-pci@...r.kernel.org, open list <linux-kernel@...r.kernel.org>
Subject: Re: extra large DMA buffer for PCI-E device under UIO

On Tue, Nov 22, 2011 at 06:54:02PM +0100, Hans J. Koch wrote:
> On Tue, Nov 22, 2011 at 07:37:23PM +0200, Michael S. Tsirkin wrote:
> [...]
> > > Or am I better off with a UIO solution?
> > 
> > You should probably write a proper kernel driver, not a UIO one.
> > your kernel driver would have to prevent the device fom DMA into memory
> > outside the allocated range, even if userspace is malicious.
> > That's why UIO is generally not recommended for PCI devices that do DMA.
> 
> When UIO was designed, the main goal was the ability to handle interrupts
> from userspace. There was no requirement for DMA. In fact, in five years I
> didn't get one real world device on my desk that needed it. That doesn't
> mean there are no such devices. Adding DMA support to the UIO core was
> discussed several times but noone ever did it. Ideas are still welcome...
> 
> If parts of the driver should be in userspace, you should really try
> to extend the UIO core instead of re-implementing UIO functionality in
> a "proper kernel driver".
> 
> Thanks,
> Hans

Right, I really meant put all of the driver in the kernel.
If parts are in userspace, and device can do DMA,
you are faced with the problem as userspace suddenly
can access arbitrary memory through the device.

-- 
MST
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