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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

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
> c) domU will not anytime update the BARs, if it does then it is a fault, 
> till we decide how to handle it

As Julien has noted pciback already deals with this correctly, because
sizing a BAR involves a write, it implementes a scheme which allows
either the hardcoded virtual BAR to be written or all 1s (needed for
size detection).

> 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)

In fact I'm now (after Julien pointed out the current behaviour of
pciback) in favour of (1), although I'm not sure if Stefano is too.

(I was never in favour of (2), FWIW, I previously was in favour of (3)
which is like (2) except pciback makes the hypervcall to map the virtual
bars to the guest, I'd still favour that over (2) but (1) is now my
preference)

Ian.


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