linux-kernel - Re: [RFC v6][PATCH 4/4] intel_txt: force IOMMU on for Intel(R) TXT launch

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Date:	Sun, 5 Jul 2009 22:26:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pavel Machek <pavel@....cz>
Cc:	Joseph Cihula <joseph.cihula@...el.com>,
	linux-kernel@...r.kernel.org, arjan@...ux.intel.com, hpa@...or.com,
	andi@...stfloor.org, chrisw@...s-sol.org, jmorris@...ei.org,
	jbeulich@...ell.com, peterm@...hat.com, gang.wei@...el.com,
	shane.wang@...el.com
Subject: Re: [RFC v6][PATCH 4/4] intel_txt: force IOMMU on for Intel(R) TXT
	launch


* Pavel Machek <pavel@....cz> wrote:

> On Fri 2009-07-03 10:21:11, Ingo Molnar wrote:
> > 
> > * Pavel Machek <pavel@....cz> wrote:
> > 
> > > 
> > > On Tue 2009-06-30 19:31:10, Joseph Cihula wrote:
> > > > The tboot module will DMA protect all of memory in order to ensure the that
> > > > kernel will be able to initialize without compromise (from DMA).  Consequently,
> > > > the kernel must enable Intel(R) Virtualization Technology for Directed I/O
> > > > (VT-d or Intel IOMMU) in order to replace this broad protection with the
> > > > appropriate page-granular protection.  Otherwise DMA devices will be unable
> > > > to read or write from memory and the kernel will eventually panic.
> > > > 
> > > > Because runtime IOMMU support is configurable by command line options, this
> > > > patch will force it to be enabled regardless of the options specified, and will
> > > > log a message if it was required to force it on.
> > > > 
> > > > 
> > > >  dmar.c        |    7 +++++++
> > > >  intel-iommu.c |   17 +++++++++++++++--
> > > >  2 files changed, 22 insertions(+), 2 deletions(-)
> > > > 
> > > > Signed-off-by: Joseph Cihula <joseph.cihula@...el.com>
> > > > Signed-off-by: Shane Wang <shane.wang@...el.com>
> > > 
> > > NAK. Breaks user expectations, misses docs updates.
> > 
> > What's your proposed solution? If an incompatible IOMMU option 
> > is specified should the kernel to disable TXT and panic?
> 
> Yes.
> 
> ...and whether we decide one way or another, it needs to be 
> documented.a

But the user already specified another thing as well: that we should 
boot with TXT.

So we have conflicting user options. Wouldnt it be the proper 
engineering solution to print a warning about the incompatible IOMMU 
option and disable it, but not crash the bootup? We generally prefer 
to boot up.

Anyway, this is a small detail clearly.

	Ingo
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