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Re: [Xen-devel] Sharing MMIO page with DomU on a system with no IOMMU

On Tue, 2012-10-16 at 08:08 +0100, Jan Beulich wrote:
> >>> On 15.10.12 at 18:50, Martin Roth <gaumless@xxxxxxxxx> wrote:
> > I'm trying to share an MMIO page from Dom0 running linux with a DomU 
> > running an RTOS so that I can access the GPIO registers directly. The 
> > system I need to get this working on does not have an IOMMU. I'm aware 
> > of the security issues of sharing this area, but don't believe it's an 
> > issue in this case.  I've got a Linux module set up and can share memory 
> > between the two domains using the grant table, but I haven't been able 
> > to get this to work for the MMIO page.
> > 
> > Can anyone give me a pointer on how I should go about doing this?  I 
> > need the area to actually be shared between both domains, not simply 
> > owned by the RTOS.
> Looks like you have to assign the PCI device providing the GPIOs
> to the guest, as there doesn't currently appear to be a way to
> allow guest access to individual MMIO pages (lacking the MMIO
> counterpart to the iports= and irqs= domain config settings).
> Ian, Ian - am I overlooking something?

iomem= support was committed to xl in xen-unstable last week or the week
before. I think it already existed in xend.

> And of course - sharing the MMIO page between Dom0 and guest
> is a questionable intention in any case - you rather want it owned
> by the DomU, and Dom0 then shouldn't access it at all. Otherwise,
> how would you expect accesses to be coordinated?
> Jan

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