Xen devel - May 2008 - PCI Passthrough using GART on AMD CPUs?

Hey everyone,

I realize that people asking about PCI passthrough has been thoroughly beaten to
death many a time, however I couldn''t find the answer to these
questions
anywhere (and I''d really like to know :P) so please just bare with me.

I know the GART built into AMD chipsets can act as a "mini IOMMU" and
do DMA
translation but not protection/isolation. However, last I checked PCI
passthrough only worked on Xen 3.2 (unstable) if a VT-d supporting chipset was
detected.

So what I wanted to know:

1) If PCI passthrough can be done using GART, then is this being worked on? Also
can PCI hotplug work as with VT-d?
2) Are there any technological limitations which prevent the GART from being
used with PCI passthrough or is this just not coded?
2) If it is not coded, is it a matter of someone not wanting to do this or is it
a matter of such a patch never being merged into mainline because of potential
stability issues due to lack of isolation?

I would like to know since AMD''s real IOMMU won''t be in
chipsets until 2010 and
you can''t get features like SLI on Intel''s chipsets. If I can
use the GART on a
Phenom say, then I can get a nVidia SLI board and have the benefits of the more
advanced features already present in the Phenom like RVI.

What do you guys think? I really appreciate your work - keep it up!

Jeff


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Also, I apologize if I should have posted this in one of the other groups like
xen.user.

Jeff


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> Also can PCI hotplug work as with VT-d?
Yes. VT-d hotplug has been supported. docs/misc/vtd.txt tells you how to
use it.

Randy (Weidong)


Jeff Williams wrote:
> Hey everyone,
> 
> I realize that people asking about PCI passthrough has been
> thoroughly beaten to death many a time, however I couldn''t find
the
> answer to these questions anywhere (and I''d really like to know
:P)
> so please just bare with me. 
> 
> I know the GART built into AMD chipsets can act as a "mini IOMMU"
and
> do DMA translation but not protection/isolation. However, last I
> checked PCI passthrough only worked on Xen 3.2 (unstable) if a VT-d
> supporting chipset was detected.
> 
> So what I wanted to know:
> 
> 1) If PCI passthrough can be done using GART, then is this being
> worked on? Also can PCI hotplug work as with VT-d?
> 2) Are there any technological limitations which prevent the GART
> from being used with PCI passthrough or is this just not coded?
> 2) If it is not coded, is it a matter of someone not wanting to do
> this or is it a matter of such a patch never being merged into
> mainline because of potential stability issues due to lack of
> isolation? 
> 
> I would like to know since AMD''s real IOMMU won''t be in
chipsets
> until 2010 and you can''t get features like SLI on Intel''s
chipsets.
> If I can use the GART on a Phenom say, then I can get a nVidia SLI
> board and have the benefits of the more advanced features already
> present in the Phenom like RVI. 
> 
> What do you guys think? I really appreciate your work - keep it up!
> 
> Jeff
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

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Han, Weidong <weidong.han <at> intel.com> writes:
> 
> > Also can PCI hotplug work as with VT-d?
> 
> Yes. VT-d hotplug has been supported. docs/misc/vtd.txt tells you how to
> use it.
> 
> Randy (Weidong)
> 
Actually I''ve read this already but thanks. I was wondering if PCI
hotplug
could be make to work with the GART mini-IOMMU on the AMD platform instead of 
using VT-d.


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we don''t have that kind of environment, but the code is designed to
support both platform.
-Xin 
>-----Original Message-----
>From: xen-devel-bounces@lists.xensource.com 
>[mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of 
>Jeff Williams
>Sent: 2008年5月16日 9:28
>To: xen-devel@lists.xensource.com
>Subject: [Xen-devel] Re: PCI Passthrough using GART on AMD CPUs?
>
>Han, Weidong <weidong.han <at> intel.com> writes:
>
>> 
>> > Also can PCI hotplug work as with VT-d?
>> 
>> Yes. VT-d hotplug has been supported. docs/misc/vtd.txt 
>tells you how to
>> use it.
>> 
>> Randy (Weidong)
>> 
>
>Actually I''ve read this already but thanks. I was wondering if 
>PCI hotplug 
>could be make to work with the GART mini-IOMMU on the AMD 
>platform instead of 
>using VT-d.
>
>
>_______________________________________________
>Xen-devel mailing list
>Xen-devel@lists.xensource.com
>http://lists.xensource.com/xen-devel
>
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Hi,

I''ve found an old post about GART as mini-IOMMU. Please see below.
As far as I read, the patch was not checked in because it was not
complete. If the code comes to the main tree, there is no reason
we don''t support PCI hotplug for GART.

http://xen.markmail.org/message/ylcrn2qgvgzlpajd

Thanks,
  Yosuke

Li, Xin B wrote:
> we don''t have that kind of environment, but the code is designed
to support both platform.
> -Xin 
> 
>> -----Original Message-----
>> From: xen-devel-bounces@lists.xensource.com 
>> [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of 
>> Jeff Williams
>> Sent: 2008年5月16日 9:28
>> To: xen-devel@lists.xensource.com
>> Subject: [Xen-devel] Re: PCI Passthrough using GART on AMD CPUs?
>>
>> Han, Weidong <weidong.han <at> intel.com> writes:
>>
>>>> Also can PCI hotplug work as with VT-d?
>>> Yes. VT-d hotplug has been supported. docs/misc/vtd.txt 
>> tells you how to
>>> use it.
>>>
>>> Randy (Weidong)
>>>
>> Actually I''ve read this already but thanks. I was wondering if
>> PCI hotplug 
>> could be make to work with the GART mini-IOMMU on the AMD 
>> platform instead of 
>> using VT-d.
>>
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xensource.com
>> http://lists.xensource.com/xen-devel
>>
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel


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The functionality of that patch is to allow dom0 to use the GART as a target
aperture for 32-bit PCI devices to access memory above 4GB. The GART cannot
possibly implement the functionality of the new AMD or Intel IOMMUs, as it
does not intercept *every* DMA access issued by guest-controlled devices.
The patch you would like to see is an impossibility.

 -- Keir

On 16/5/08 05:05, "Yosuke Iwamatsu" <y-iwamatsu@ab.jp.nec.com>
wrote:
> Hi,
> 
> I''ve found an old post about GART as mini-IOMMU. Please see below.
> As far as I read, the patch was not checked in because it was not
> complete. If the code comes to the main tree, there is no reason
> we don''t support PCI hotplug for GART.
> 
> http://xen.markmail.org/message/ylcrn2qgvgzlpajd
> 
> Thanks,
>   Yosuke
> 
> Li, Xin B wrote:
>> we don''t have that kind of environment, but the code is
designed to support
>> both platform.
>> -Xin 
>> 
>>> -----Original Message-----
>>> From: xen-devel-bounces@lists.xensource.com
>>> [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of
>>> Jeff Williams
>>> Sent: 2008年5月16日 9:28
>>> To: xen-devel@lists.xensource.com
>>> Subject: [Xen-devel] Re: PCI Passthrough using GART on AMD CPUs?
>>> 
>>> Han, Weidong <weidong.han <at> intel.com> writes:
>>> 
>>>>> Also can PCI hotplug work as with VT-d?
>>>> Yes. VT-d hotplug has been supported. docs/misc/vtd.txt
>>> tells you how to
>>>> use it.
>>>> 
>>>> Randy (Weidong)
>>>> 
>>> Actually I''ve read this already but thanks. I was
wondering if
>>> PCI hotplug 
>>> could be make to work with the GART mini-IOMMU on the AMD
>>> platform instead of
>>> using VT-d.
>>> 
>>> 
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@lists.xensource.com
>>> http://lists.xensource.com/xen-devel
>>> 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xensource.com
>> http://lists.xensource.com/xen-devel
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel


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Keir Fraser <keir.fraser <at> eu.citrix.com> writes:
> 
> The functionality of that patch is to allow dom0 to use the GART as a
target
> aperture for 32-bit PCI devices to access memory above 4GB. The GART cannot
> possibly implement the functionality of the new AMD or Intel IOMMUs, as it
> does not intercept *every* DMA access issued by guest-controlled devices.
> The patch you would like to see is an impossibility.
> 
>  -- Keir
> 
Thank you Keir for clarifying this. I guess I''ll just have to wait, or
get an
Intel VT-d board. 

Too bad Skulltrail is still so expensive..

Jeff




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