linux-kernel - Re: [PATCH V1 4/6] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

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Date:   Tue, 3 May 2022 20:09:32 +0300
From:   Oleksandr <olekstysh@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     xen-devel@...ts.xenproject.org,
        virtualization@...ts.linux-foundation.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Jason Wang <jasowang@...hat.com>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Julien Grall <julien@....org>, Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH V1 4/6] dt-bindings: Add xen,dev-domid property
 description for xen-grant DMA ops


On 03.05.22 00:59, Rob Herring wrote:

Hello Rob


> On Fri, Apr 22, 2022 at 07:51:01PM +0300, Oleksandr Tyshchenko wrote:
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>>
>> Introduce Xen specific binding for the virtualized device (e.g. virtio)
>> to be used by Xen grant DMA-mapping layer in the subsequent commit.
>>
>> This binding indicates that Xen grant mappings scheme needs to be
>> enabled for the device which DT node contains that property and specifies
>> the ID of Xen domain where the corresponding backend resides. The ID
>> (domid) is used as an argument to the grant mapping APIs.
>>
>> This is needed for the option to restrict memory access using Xen grant
>> mappings to work which primary goal is to enable using virtio devices
>> in Xen guests.
>>
>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>> ---
>> Changes RFC -> V1:
>>     - update commit subject/description and text in description
>>     - move to devicetree/bindings/arm/
>> ---
>>   .../devicetree/bindings/arm/xen,dev-domid.yaml     | 37 ++++++++++++++++++++++
>>   1 file changed, 37 insertions(+)
>>   create mode 100644 Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
>>
>> diff --git a/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
>> new file mode 100644
>> index 00000000..ef0f747
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
>> @@ -0,0 +1,37 @@
>> +# SPDX-License-Identifier: (GPL-2.0-only or BSD-2-Clause)
>> +%YAML 1.2
>> +---
>> +$id: http://devicetree.org/schemas/arm/xen,dev-domid.yaml#
>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>> +
>> +title: Xen specific binding for the virtualized device (e.g. virtio)
>> +
>> +maintainers:
>> +  - Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>> +
>> +select: true
> Do we really need to support this property everywhere?

 From my understanding - yes.

As, I think, any device node describing virtulized device in the guest 
device tree can have this property.  Initially (in the RFC series) the 
"solution to restrict memory access using Xen grant mappings" was 
virtio-specific.

Although the support of virtio is a primary target of this series, we 
decided to generalize this work and expand it to any device [1]. So the 
Xen grant mappings scheme (this property to be used for) can be 
theoretically used for any device emulated by the Xen backend.


>> +
>> +description:
>> +  This binding indicates that Xen grant mappings scheme needs to be enabled
>> +  for that device and specifies the ID of Xen domain where the corresponding
>> +  device (backend) resides. This is needed for the option to restrict memory
>> +  access using Xen grant mappings to work.
>> +
>> +properties:
>> +  xen,dev-domid:
>> +    $ref: /schemas/types.yaml#/definitions/uint32
>> +    description:
>> +      The domid (domain ID) of the domain where the device (backend) is running.
>> +
>> +additionalProperties: true
>> +
>> +examples:
>> +  - |
>> +    virtio_block@...0 {
> virtio@...0

ok, will change


>
>> +            compatible = "virtio,mmio";
>> +            reg = <0x3000 0x100>;
>> +            interrupts = <41>;
>> +
>> +            /* The device is located in Xen domain with ID 1 */
>> +            xen,dev-domid = <1>;
> This fails validation:
>
> Documentation/devicetree/bindings/arm/xen,dev-domid.example.dtb: virtio_block@...0: xen,dev-domid: [[1]] is not of type 'object'
>          From schema: /home/rob/proj/git/linux-dt/Documentation/devicetree/bindings/virtio/mmio.yaml

Thank you for pointing this out, my fault, I haven't "properly" checked 
this before. I think, we need to remove "compatible = "virtio,mmio"; here


diff --git a/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml 
b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
index 2daa8aa..d2f2140 100644
--- a/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
+++ b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
@@ -28,7 +28,7 @@ additionalProperties: true
  examples:
    - |
      virtio_block@...0 {
-            compatible = "virtio,mmio";
+            /* ... */
              reg = <0x3000 0x100>;
              interrupts = <41>;



>
> The property has to be added to the virtio/mmio.yaml schema. If it is
> not needed elsewhere, then *just* add the property there.

As I described above, the property is not virtio specific and can be 
used for any virtualized device for which Xen grant mappings scheme 
needs to be enabled (xen-grant DMA-mapping layer).


[1] 
https://lore.kernel.org/xen-devel/alpine.DEB.2.22.394.2204181202080.915916@ubuntu-linux-20-04-desktop/


>
> Rob

-- 
Regards,

Oleksandr Tyshchenko