Re: PCI Passthrough ARM Design : Draft1

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Wed, 17 Jun 2015, 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
> > 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)

OK, let's go with (1).