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Re: [Xen-devel] PCI Passthrough ARM Design : Draft1





On Wednesday 17 June 2015 06:43 AM, Ian Campbell wrote:
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.
Does the Flow below captures all points
a) When assigning a device to domU, toolstack creates a node in per device directory with virtual BAR address/size

Option1:
b) toolstack using some hypercall ask xen to create p2m mapping { virtual BAR : physical BAR } for domU c) domU will not anytime update the BARs, if it does then it is a fault, till we decide how to handle it d) when domU queries BAR address from pci-back the virtual BAR address is provided.

Option2:
b) domU will not anytime update the BARs, if it does then it is a fault, till we decide how to handle it c) when domU queries BAR address from pci-back the virtual BAR address is provided.
d) domU sends a hypercall to map virtual BARs,
e) xen pci code reads the BAR and maps { virtual BAR : physical BAR } for domU

Which option is better I think Ian is for (2) and Stefano may be (1)

Ian.


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