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Re: [Xen-devel] [Query] Flow of PCI device dedicated to a domU

On Tue, 2014-07-22 at 16:56 +0100, Simon Martin wrote:
> Hello Stefano,
> > The basic mechanism is that Xen assigns the PCI root complex/host
> > bridge to Dom0. Dom0 walks the bus and finds out the devices available.
> > When the user assigns a PCI device to a VM, the toolstack (xl/libxl)
> > issues hypercalls to remap interrupts and MMIO regions of that device
> > into the guest VM.
> I am currently trying writing my own PCI front to access directly a
> PCI device from my PV guest.
> If I read this correctly you are saying that the MMIO region is
> already mapped into my domain by xl? So when I go and read the BAR
> register from the PCI configuration I can just go and access it
> directly or do I have to update the VA mapping?

Stefano meant mapped in the p2m which under ARM or x86 HVM still means
you need to make a virtual mapping of those PFNs.

Under X86 PV "mapping" in this context really just means giving
permission to map the underlying MFNs, so you still need to create a
virtual mapping but they don't exist in your p2m until you put them


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