linux-kernel - Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

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Date:   Wed, 18 May 2022 19:06:08 +0300
From:   Oleksandr <olekstysh@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     xen-devel <xen-devel@...ts.xenproject.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@...ts.linux-foundation.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Jason Wang <jasowang@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        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>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property
 description for xen-grant DMA ops


On 18.05.22 17:32, Arnd Bergmann wrote:


Hello Arnd


> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@...il.com> wrote:
>> 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@...0 {
    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