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

[Manish Jaggi <mjaggi@xxxxxxxxxxxxxxxxxx>]

On Friday 26 June 2015 01:02 PM, Ian Campbell wrote:
> On Fri, 2015-06-26 at 07:37 +0530, Manish Jaggi wrote:
> > On Thursday 25 June 2015 10:56 PM, Konrad Rzeszutek Wilk wrote:
> > > On Thu, Jun 25, 2015 at 01:21:28PM +0100, Ian Campbell wrote:
> > > > On Thu, 2015-06-25 at 17:29 +0530, Manish Jaggi wrote:
> > > > > On Thursday 25 June 2015 02:41 PM, Ian Campbell wrote:
> > > > > > 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.
> > > > > Yes, the point is the pciback driver reads the physical BAR regions on
> > > > > request from domU.
> > > > > So it sends a hypercall to map the physical bars into stage2 translation
> > > > > for the domU through xen.
> > > > > Xen would use the holes left in IPA for MMIO.
> > > > I still think it is the toolstack which should do this, that's whewre
> > > > these sorts of layout decisions belong.
> > can the xl tools read pci conf space ?
> Yes, via sysfs (possibly abstracted via libpci) . Just like lspci and
> friends do.
>
> > Using some xen hypercall or a xl-dom0 ioctl ?
> No, using normal pre-existing Linux functionality.
>
> > If not then there is no otherway but xenpciback
> >
> > Also I need to introduce a hypercall which would tell toolkit the
> > available holes for virtualBAR mapping.
> > Much simpler is let xen allocate a virtualBAR and return to the caller.
> > > At init - sure. But when the guest is running and doing those sort
> > > of things. Unless you want guest -> pciback -> xenstore -> libxl ->
> > > hypercall -> send ack on xenstore -> pciback -> guest.
> > >
> > > That would entail adding some pcibkack -> user-space tickle mechanism
> > > and another back. Much simpler to do all of this in xenpciback I think?
> > I agree. If the xenpciback sends a hypercall whenever a BAR read access,
> > the mapping
> > in xen would already have been done, so xen would simply be doing
> > PA->IPA lookup.
> > No xenstore lookup is required.
> The xenstore read would happen once on device attach, at the same time
> you are reading the rest of the dev-NNN stuff relating to the just
> attached device.
>
> Doing a xenstore transaction on every BAR read would indeed be silly and
> doing a hypercall would not be much better. There is no need for either
> a xenstore read or a hypercall during the cfg space access itself, you
> just read the value from a pciback datastructure.
>
> Add to that the fact that any new hypercall made from dom0 needs to be
> added as a stable interface I can't see any reason to go with such a
> model.
I think you are overlooking a point which is "From what region the 
virtual BAR be allocated ?"
One way is for xen to keep a hole for domains where the bar regions be 
mapped. This is not there as of now.
How would the tools know about this hole ?
A domctl is required ?
For this reason I was suggesting a hypercall to xen to map the physical 
BARs and return the virtualBARs.
> Ian.
>
>
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