Re: [Xen-devel] Direct Device Assignment in Xen DomU.

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 12 Aug 2014, manish jaggi wrote:
> Hi,
> 
> On our platform there are 2 uarts. Is there a feature in Xen to assign
> a uart to DomU.
> As uart is not a bus master IOMMU is not required.
> 
> I couldnt find anything googling for this.

Today Xen cannot distinguish between bus mastering devices and non bus
mastering devices. Therefore without an IOMMU in front of it, Xen is
going to assume that it wouldn't be safe to assign the device to the
guest.

However Xen provide ways to remap generic mmio regions and interrupts
into a guest, so you could simply do that to get the uart in your domU.
Just add:

iomem=["0x10000,2"]
irqs=[109]

to your VM config file, where 109 is the uart irq, 0x100 the start mfn
and 2 the number of pages to remap. Give a look at man xl.cfg for more
info. You also need to take care of disabling the second uart in dom0
and/or Xen.

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