Re: PCI Passthrough ARM Design : Draft1

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-06-17 at 13:58 +0100, Stefano Stabellini wrote:
> Yes, pciback is already capable of doing that, see
> drivers/xen/xen-pciback/conf_space.c
> 
> > I am not sure if the pci-back driver can query the guest memory map. Is there an existing hypercall ?
> 
> No, that is missing.  I think it would be OK for the virtual BAR to be
> initialized to the same value as the physical BAR.  But I would let the
> guest change the virtual BAR address and map the MMIO region wherever it
> wants in the guest physical address space with
> XENMEM_add_to_physmap_range.

I disagree, given that we've apparently survived for years with x86 PV
guests not being able to right to the BARs I think it would be far
simpler to extend this to ARM and x86 PVH too than to allow guests to
start writing BARs which has various complex questions around it.

All that's needed is for the toolstack to set everything up and write
some new xenstore nodes in the per-device directory with the BAR
address/size.

Also most guests apparently don't reassign the PCI bus by default, so
using a 1:1 by default and allowing it to be changed would require
modifying the guests to reasssign. Easy on Linux, but I don't know about
others and I imagine some OSes (especially simpler/embedded ones) are
assuming the firmware sets up something sane by default.

Ian.