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Re: RFC: PCI devices passthrough on Arm design proposal

On 17.07.20 19:18, Julien Grall wrote:

Hello Bertrand

[two threads with the same name are shown in my mail client, so not completely sure I am asking in the correct one]

On 17/07/2020 17:08, Roger Pau Monné wrote:
On Fri, Jul 17, 2020 at 03:51:47PM +0000, Bertrand Marquis wrote:

On 17 Jul 2020, at 17:30, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:

On Fri, Jul 17, 2020 at 03:23:57PM +0000, Bertrand Marquis wrote:

On 17 Jul 2020, at 17:05, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:

On Fri, Jul 17, 2020 at 02:49:20PM +0000, Bertrand Marquis wrote:

On 17 Jul 2020, at 16:41, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:

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?

We will add some new definitions for the ECAM regions in the guest physmap declared in xen (include/asm-arm/config.h)

I think I'm confused, but that file doesn't contain anything related
to the guest physmap, that's the Xen virtual memory layout on Arm

Does this somehow relate to the physical memory map exposed to guests
on Arm?

Yes it does.
We will add new definitions there related to VPCI to reserve some areas for the VPCI ECAM and the IOMEM areas.

Yes, that's completely fine and is what's done on x86, but again I
feel like I'm lost here, this is the Xen virtual memory map, how does
this relate to the guest physical memory map?

Sorry my bad, we will add values in include/public/arch-arm.h, wrong header :-)

Oh right, now I see it :).

Do you really need to specify the ECAM and MMIO regions there?

You need to define those values somewhere :). The layout is only shared between the tools and the hypervisor. I think it would be better if they are defined at the same place as the rest of the layout, so it is easier to rework the layout.


I would like to clarify regarding an IOMMU driver changes which should be done to support PCI pass-through properly.

Design document mentions about SMMU, but Xen also supports IPMMU-VMSA (under tech preview now). It would be really nice if the required support is extended to that kind of IOMMU as well.

May I clarify what should be implemented in the Xen driver in order to support PCI pass-through feature on Arm? Should the IOMMU H/W be "PCI-aware" for that purpose?


Oleksandr Tyshchenko



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