Re: [Xen-users] PCI Passthrough

[Paul Schulze <avlex@xxxxxxx>]

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Hi Mark,

Am 25.05.2008 um 02:22 schrieb Mark Williamson:

> > I have already heard about IOMMU being implemented in Intel CPUs (or
> > probably the North Bridge, because as I hear that is where the Memory
> > Controller is located) only, however, as far as I can see AMD isn't
> > quiet there yet (I hear they postponed it to 2009 again, almost
> > reminds me of GNU/Hurd). However, that is one of the main problems I
> > am facing: Intel does not offer a suitable basis for low power
> > systems with desktop performance.
> How low do you need the power consumption to be?  Intel's recent  
> chips aren't
> as scarily "hungrier than everything else" as they were back in the  
> old
> Pentium 4 days, although I guess the "normal" power consumption has  
> gone up
> since then too...
Thats a good question, as low as possible. For the platform I am  
currently working on that means something around 50W, 70W at the most  
when "idling" (might sound a little too enthusiastic, but its the  
general target). I assume that most of the tasks I give it will leave  
the machine more or less idling (below 10% on the CPU), even  
virtualized, which only works with a CPU with enough power, so it can  
clock down if the full speed isn't needed. The low power requirement  
practically disqualifies anything without onboard graphics, since the  
system has to deal with at least 2 HDDs and the network and wifi  
hardware, not to mention the TV tuner card. I had a look at the X3500  
IGP on Intels new chipset, but it seems like it still is no  
alternative to the AMD 780G and I strongly suspect the nForce780a  
chipset to not support VT-d (besides it being not available for Intel  
CPUs at the moment). The requirements for extension slots practically  
dismiss all the available mainboards for mobile Intel CPUs, since  
those are either MicroATX or ITX too. I would really have prefered an  
Intel CPU, but the current situation doesnt offer much of a choice.
> I'd also note that there are now tiny motherboards based on Intel's  
> Atom CPU
> for very low power applications, although they won't give you the  
> desktop
> performance you want.  You might want to consider splitting some of  
> the
> functionality of this system off onto a minimal box like that so the
> powerful, hungry desktop hardware can be powered off completely  
> when not
> required?
And there is the problem, I want one box that can handle it all,  
because first of all, two boxes are more expensive and second of all,  
if I need the desktops power over night, it will still have to run,  
ultimately leaving me with two maschines worth of power drawn and  
noise produced. In my opinion, for this case a single system can meet  
the requirements perfectly well and scales better for this kind of  
application.
Intel's Atom CPU is out of the question though, It will probably not  
be able to handle all the services I would require from the platform  
at peek times. That includes HDTV decoding and reencoding for  
possible MythTV clients (3 at the moment), which alone would more or  
less kill the whole system. Having this part handled by the Desktop  
system is impractical too, because whomever wants to use the MythTV  
server over network would have to turn on the Desktop first (and the  
current server provides resources for people outside my apartment  
too... and sucks power like crazy doing it because of a defect, which  
is why its going to be replaced).
> > I already looked far and wide for a
> > suitable CPU + Mainboard combination with low power consumption and
> > onboard 3D graphics that are worth something and I'm sorry to say,
> > but Intel's are definitively not (compared to the AMD 4x50e CPUs with
> > AMD780G chipsets at least). So I am basically bound to AMD for this
> > particular project.
> OK.  Well if you have a particularly compelling need for AMD then  
> that's fine
> but it is going to be a problem for the security of PCI passthrough...
I rather wouldn't, but the alternatives in form of performance  
combined with low power consumption are less compelling and if it  
comes down to it, it is supposed to be a multimedia and home server.  
However, if possible, I would still like to close security holes,  
especially for the Firewall and Access Point VM (which is probably  
the most critical part).
> > I already looked around for clues on a software IOMMU implementation
> > too, but the only thing I could find was SWIOTLB. As I understand it,
> > this solution merely allows 32bit devices to use more than 4gb of
> > RAM, or is there a way to use it as a software IOMMU in the sense of
> > Intel VT-d too? If not, is there another way to emulate IOMMU or at
> > least protect the system from a potentially compromised privileged
> > DomU until AMD CPUs supporting this feature are available?
> I'm afraid there's no practical way of doing untrusted PCI passthrough
> securely without having an IOMMU in hardware.  Without special  
> hardware to
> enforce memory access controls, a domain with direct access to a  
> PCI card
> I'm afraid the "solution" to running untrusted operating systems is to
> virtualise the devices too - using virtual network, graphics, etc  
> devices,
> it's possible to provide more stringent controls on what they can /  
> can't do
> than if you've given a guest *real* hardware.  Unfortunately, this  
> doesn't
> seem to be a particularly good fit for most of what you want to do :-(
> > And am I
> > correct to assume that a possible feature for AMD CPUs will possibly
> > not need support from the chipset, because the Memory Controller is
> > located on the CPU?
> That sounds sane but I don't know enough about the AMD platform  
> (and their
> corporate plans!) to answer that one reliably.
I know what you mean, I am only guessing here too. And I will  
probably have to stake a whole system on that guess unless another  
solution pops up. It will be a real pain to secure the VMs though.
> > I hope someone can help me out of my confusion,
> I hope that clears things up a bit.  Sorry if it's not really the  
> ideal answer
> for you though.
It does, and don't worry, I wasn't looking for an ideal answer  
anyways. If that would exist, the whole project wouldn't interest me  
at all :) . Besides, if there were a simple way, we would have linux  
distros for sandboxed multimedia systems already. It really wouldn't  
be fun that way.
Thanks again,


Paul.

- --
Paul Schulze
avlex@xxxxxxx
Public Key: http://solaris-net.dyndns.org/keys/key_avlex.asc

"Making mistakes is human,
but to really fuck things up you need Computers"

> Cheers,
> Mark
>
> --
> Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/ 
> ~maw48/pmpu/)
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