From: Ingo Molnar <mingo@kernel.org>
To: Joerg Roedel <jroedel@suse.de>
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
Date: Sat, 6 May 2017 11:48:45 +0200 [thread overview]
Message-ID: <20170506094845.djhjiugvtuvqmbpc@gmail.com> (raw)
In-Reply-To: <20170505084004.GM5077@suse.de>
* Joerg Roedel <jroedel@suse.de> wrote:
> Hi Ingo,
>
> On Fri, May 05, 2017 at 08:59:20AM +0200, Ingo Molnar wrote:
> > * Joerg Roedel <jroedel@suse.de> wrote:
>
> > > 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.
> >
> > Yes, that makes sense - but in this case it would be far more user friendly to
> > make it a sysctl, not a boot option. This is also much more manageable for
> > distributions and also allows it to be more easily turned into a security policy
> > feature.
> >
> > New boot options should be for debugging hacks in essence - any serious hardware
> > configuration should be done via more user-friendly methods.
>
> I agree in general that a sysctl would be more user-friendly. But the
> problem is that enabling/disabling the IOMMU is a boot-time option that
> can't be changed at runtime.
>
> That is because this decission defines how the bus addresses are mapped
> to physical addresses through the dma-api. When the iommu is disabled,
> it is just a 1-1 mapping, but when it is enabled a physical address
> could end up on any address in the bus address space.
>
> Once drivers are loaded that allocate those addresses we can't change
> the mappings anymore as disabling the iommu would do.
Ok - that makes sense - I withdraw my objections:
Acked-by: Ingo Molnar <mingo@kernel.org>
Thanks,
Ingo
prev parent reply other threads:[~2017-05-06 9:48 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-26 16:18 [PATCH V2] x86/tboot: add an option to disable iommu force on Shaohua Li
2017-04-26 21:59 ` Joerg Roedel
2017-04-27 6:52 ` Ingo Molnar
2017-04-28 22:07 ` Joerg Roedel
2017-04-27 6:51 ` Ingo Molnar
2017-04-27 8:42 ` Joerg Roedel
2017-04-27 14:49 ` Shaohua Li
2017-04-27 15:18 ` Joerg Roedel
2017-04-27 15:41 ` Shaohua Li
2017-04-27 16:04 ` Joerg Roedel
2017-05-05 6:59 ` Ingo Molnar
2017-05-05 8:40 ` Joerg Roedel
2017-05-06 9:48 ` Ingo Molnar [this message]
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