Re: [Xen-devel] Questions about direct hardware access, DMA, etc

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Hi folks, this is my first post to the mailing list.  I have a general
> question about the state of various efforts aimed at granting a domU
> physical access to a hardware device like a NIC, graphics card, etc.
> I generally understand the challenges this poses to a hypervisor in
> terms of interrupts and DMA.  I am specifically wondering about what
> options are available when running non-Paravirtualized OSes (with
> VTX/SVM)
>
> I guess Xen has supported "driver domains" for awhile...but I assume
> these don't work if the device uses DMA?  Or do they only work for PV
> guests?

Only PV guests.

Well, in principle it would be possible to make a non-DMA device pass through 
to an HVM guest fairly straightforwardly, although to the best of my 
knowledge this is not currently done.  You could possibly tell the Qemu 
device model to e.g. plumb the guest's serial console to /dev/ttyS0 and thus 
get some serial port IO (but not direct access).

> What projects/repositories are there (on
> http://xenbits.xensource.com/ext or elsewhere) that address granting a
> non-PV DomU access to a device...either with a hardware IOMMU or
> without one?  I saw some slides on Neocleus that looked
> interesting...my take was that they are giving a non-PV DomU access to
> a device in the absence of a hardware IOMMU, is that roughly correct?
> Is the Neocleus project in any particular repository now?

Yes, that's what they're trying to do.  They posted patches to the mailing 
list during the last couple of months, which you may want to take a look at.  
I'm not sure there's a public tree but they're posting patches for review and 
incorporating feedback.

Other possibilities:

Some patches for Intel's IOMMU have been posted and may have actually been 
checked in

Some AMD work was done on using the GART as a translating (but not protecting) 
IOMMU for passing devices to guests.  I'm not sure what happened to that 
project as I've not heard about it for a while.

Hope that helps,
Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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