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

[Manish Jaggi <mjaggi@xxxxxxxxxxxxxxxxxx>]

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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