Re: RFC: PCI devices passthrough on Arm design proposal

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jul 17, 2020 at 02:34:55PM +0000, Bertrand Marquis wrote:
> 
> 
> > On 17 Jul 2020, at 16:06, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> > 
> > On 17.07.2020 15:59, Bertrand Marquis wrote:
> >> 
> >> 
> >>> On 17 Jul 2020, at 15:19, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >>> 
> >>> On 17.07.2020 15:14, Bertrand Marquis wrote:
> >>>>> On 17 Jul 2020, at 10:10, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >>>>> On 16.07.2020 19:10, Rahul Singh wrote:
> >>>>>> # Emulated PCI device tree node in libxl:
> >>>>>> 
> >>>>>> Libxl is creating a virtual PCI device tree node in the device tree to 
> >>>>>> enable the guest OS to discover the virtual PCI during guest boot. We 
> >>>>>> introduced the new config option [vpci="pci_ecam"] for guests. When 
> >>>>>> this config option is enabled in a guest configuration, a PCI device 
> >>>>>> tree node will be created in the guest device tree.
> >>>>> 
> >>>>> I support Stefano's suggestion for this to be an optional thing, i.e.
> >>>>> there to be no need for it when there are PCI devices assigned to the
> >>>>> guest anyway. I also wonder about the pci_ prefix here - isn't
> >>>>> vpci="ecam" as unambiguous?
> >>>> 
> >>>> This could be a problem as we need to know that this is required for a 
> >>>> guest upfront so that PCI devices can be assigned after using xl. 
> >>> 
> >>> I'm afraid I don't understand: When there are no PCI device that get
> >>> handed to a guest when it gets created, but it is supposed to be able
> >>> to have some assigned while already running, then we agree the option
> >>> is needed (afaict). When PCI devices get handed to the guest while it
> >>> gets constructed, where's the problem to infer this option from the
> >>> presence of PCI devices in the guest configuration?
> >> 
> >> If the user wants to use xl pci-attach to attach in runtime a device to a 
> >> guest, this guest must have a VPCI bus (even with no devices).
> >> If we do not have the vpci parameter in the configuration this use case 
> >> will not work anymore.
> > 
> > That's what everyone looks to agree with. Yet why is the parameter needed
> > when there _are_ PCI devices anyway? That's the "optional" that Stefano
> > was suggesting, aiui.
> 
> I agree in this case the parameter could be optional and only required if not 
> PCI device is assigned directly in the guest configuration.

Where will the ECAM region(s) appear on the guest physmap?

Are you going to re-use the same locations as on the physical
hardware, or will they appear somewhere else?

Roger.