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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2015-06-25 at 13:14 +0530, Manish Jaggi wrote:
> 
> On Wednesday 17 June 2015 07:59 PM, Ian Campbell wrote:
> > On Wed, 2015-06-17 at 07:14 -0700, Manish Jaggi wrote:
> >> 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
> While implementing I think rather than the toolstack, pciback driver in 
> dom0 can send the
> hypercall by to map the physical bar to virtual bar.
> Thus no xenstore entry is required for BARs.

pciback doesn't (and shouldn't) have sufficient knowledge of the guest
address space layout to determine what the virtual BAR should be. The
toolstack is the right place for that decision to be made.

> Moreover a pci driver would read BARs only once.

You can't assume that though, a driver can do whatever it likes, or the
module might be unloaded and reloaded in the guest etc etc.

Are you going to send out a second draft based on the discussion so far?

Ian.


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel