On 18.05.22 17:32, Arnd Bergmann wrote:


Hello Arnd

On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko [off-list ref] wrote:

quoted

diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
index 10c22b5..29a0932 100644
--- a/Documentation/devicetree/bindings/virtio/mmio.yaml
+++ b/Documentation/devicetree/bindings/virtio/mmio.yaml

@@ -13,6 +13,9 @@ description:
    See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
    more details.

+allOf:
+  - $ref: /schemas/arm/xen,dev-domid.yaml#
+
  properties:
    compatible:
      const: virtio,mmio

@@ -33,6 +36,10 @@ properties:
      description: Required for devices making accesses thru an IOMMU.
      maxItems: 1

+  xen,dev-domid:
+    description: Required when Xen grant mappings need to be enabled for device.
+    $ref: /schemas/types.yaml#/definitions/uint32
+
  required:
    - compatible
    - reg

Sorry for joining the discussion late. Have you considered using the
generic iommu
binding here instead of a custom property?

I have to admit - no, I haven't. I was thinking that Xen specific 
feature should be communicated using Xen specific DT property.

  This would mean having a device
node for the grant-table mechanism that can be referred to using the 'iommus'
phandle property, with the domid as an additional argument.

I assume, you are speaking about something like the following?


xen_dummy_iommu {
    compatible = "xen,dummy-iommu";
    #iommu-cells = <1>;
};

virtio@3000 {
    compatible = "virtio,mmio";
    reg = <0x3000 0x100>;
    interrupts = <41>;

    /* The device is located in Xen domain with ID 1 */
    iommus = <&xen_dummy_iommu 1>;
};

It does not quite fit the model that Linux currently uses for iommus,
as that has an allocator for dma_addr_t space

yes (# 3/7 adds grant-table based allocator)

, but it would think it's
conceptually close enough that it makes sense for the binding.

Interesting idea. I am wondering, do we need an extra actions for this 
to work in Linux guest (dummy IOMMU driver, etc)?

          Arnd

-- 
Regards,

Oleksandr Tyshchenko


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