Date: Thu, 27 Apr 2017 10:42:07 +0200
From: Joerg Roedel <jroedel@suse.de>
To: Ingo Molnar <mingo@kernel.org>
Cc: Shaohua Li <shli@fb.com>, linux-kernel@vger.kernel.org,
        gang.wei@intel.com, hpa@linux.intel.com, kernel-team@fb.com,
        ning.sun@intel.com, srihan@fb.com, alex.eydelberg@intel.com
Subject: Re: [PATCH V2] x86/tboot: add an option to disable iommu force on
Message-ID: <20170427084207.GU5077@suse.de>
References: <1c2cadcf5cd7d19cea93c56435610e61b551bd1e.1493223474.git.shli@fb.com>
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On Thu, Apr 27, 2017 at 08:51:42AM +0200, Ingo Molnar wrote:
> > +		tboot_noforce [Default Off]
> > +			Do not force the Intel IOMMU enabled under tboot.
> > +			By default, tboot will force Intel IOMMU on, which
> > +			could harm performance of some high-throughput
> > +			devices like 40GBit network cards, even if identity
> > +			mapping is enabled.
> > +			Note that using this option lowers the security
> > +			provided by tboot because it makes the system
> > +			vulnerable to DMA attacks.
> 
> So what's the purpose of this kernel option?
> 
> It sure isn't the proper solution for correctly architectured hardware/firmware 
> (which can just choose not to expose the IOMMU!), and for one-time hacks for 
> special embedded systems or for debugging why not just add an iommu=off option to 
> force it off?

I guess that tboot requires an IOMMU to be present in order to work. It
will do initial IOMMU setup and hands the hardware over to Linux later
on.

The problem solved here is that someone wants tboot for security
reasons, but doesn't want the performance penalty of having the IOMMU
enabled and can live with the risk of an DMA attack.


Regards,

	Joerg