Re: [PATCH v4 9/9] docs: Add device tree overlay documentation

[Julien Grall <julien@xxxxxxx>]

Hi Henry,

On 23/05/2024 08:40, Henry Wang wrote:
> From: Vikram Garhwal <fnu.vikram@xxxxxxxxxx>
>
> Signed-off-by: Vikram Garhwal <fnu.vikram@xxxxxxxxxx>
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
> ---
> v4:
> - No change.
> v3:
> - No change.
> v2:
> - Update the content based on the changes in this version.
> ---
>   docs/misc/arm/overlay.txt | 99 +++++++++++++++++++++++++++++++++++++++
>   1 file changed, 99 insertions(+)
>   create mode 100644 docs/misc/arm/overlay.txt
>
> diff --git a/docs/misc/arm/overlay.txt b/docs/misc/arm/overlay.txt
> new file mode 100644
> index 0000000000..811a6de369
> --- /dev/null
> +++ b/docs/misc/arm/overlay.txt
> @@ -0,0 +1,99 @@
> +# Device Tree Overlays support in Xen
> +
> +Xen now supports dynamic device assignment to running domains,
This reads as we "support" the feature. I would prefer if we write "Xen 
expirementally supports..." or similar.
> +i.e. adding/removing nodes (using .dtbo) to/from Xen device tree, and
> +attaching/detaching them to/from a running domain with given $domid.
> +
> +Dynamic node assignment works in two steps:
> +
> +## Add/Remove device tree overlay to/from Xen device tree
> +
> +1. Xen tools check the dtbo given and parse all other user provided arguments
> +2. Xen tools pass the dtbo to Xen hypervisor via hypercall.
> +3. Xen hypervisor applies/removes the dtbo to/from Xen device tree.
> +
> +## Attach/Detach device from the DT overlay to/from domain
> +
> +1. Xen tools check the dtbo given and parse all other user provided arguments
> +2. Xen tools pass the dtbo to Xen hypervisor via hypercall.
> +3. Xen hypervisor attach/detach the device to/from the user-provided $domid by
> +   mapping/unmapping node resources in the DT overlay.
> +
> +# Examples
> +
> +Here are a few examples on how to use it.
> +
> +## Dom0 device add
> +
> +For assigning a device tree overlay to Dom0, user should firstly properly
> +prepare the DT overlay. More information about device tree overlays can be
> +found in [1]. Then, in Dom0, enter the following:
> +
> +    (dom0) xl dt-overlay add overlay.dtbo
> +
> +This will allocate the devices mentioned in overlay.dtbo to Xen device tree.
> +
> +To assign the newly added device from the dtbo to Dom0:
> +
> +    (dom0) xl dt-overlay attach overlay.dtbo 0
> +
> +Next, if the user wants to add the same device tree overlay to dom0
> +Linux, execute the following:
> +
> +    (dom0) mkdir -p /sys/kernel/config/device-tree/overlays/new_overlay
> +    (dom0) cat overlay.dtbo > 
> /sys/kernel/config/device-tree/overlays/new_overlay/dtbo
> +
> +Finally if needed, the relevant Linux kernel drive can be loaded using:
> +
> +    (dom0) modprobe module_name.ko
> +
> +## Dom0 device remove
> +
> +For removing the device from Dom0, first detach the device from Dom0:
> +
> +    (dom0) xl dt-overlay detach overlay.dtbo 0
> +
> +NOTE: The user is expected to unload any Linux kernel modules which
> +might be accessing the devices in overlay.dtbo before detach the device.
> +Detaching devices without unloading the modules might result in a crash.
> +
> +Then remove the overlay from Xen device tree:
> +
> +    (dom0) xl dt-overlay remove overlay.dtbo
> +
> +## DomU device add/remove
> +
> +All the nodes in dtbo will be assigned to a domain; the user will need
> +to prepare the dtb for the domU. For example, the `interrupt-parent` property
> +of the DomU overlay should be changed to the Xen hardcoded value `0xfde8`.
> +Below assumes the properly written DomU dtbo is `overlay_domu.dtbo`.
> +
> +User will need to create the DomU with below properties properly configured
> +in the xl config file:
> +- `iomem`
I don't quite understand how the user can specify the MMIO region if the 
device is attached after the domain is created.
> +- `passthrough` (if IOMMU is needed)
> +
> +User will also need to modprobe the relevant drivers.
> +
> +Example for domU device add:
> +
> +    (dom0) xl dt-overlay add overlay.dtbo            # If not executed before
> +    (dom0) xl dt-overlay attach overlay.dtbo $domid
Can how clarify how the MMIO will be mapped? Is it direct mapped? If so, 
couldn't this result to clash with other part of the address space (e.g. 
RAM?).
> +    (dom0) xl console $domid                         # To access $domid console
> +
> +Next, if the user needs to modify/prepare the overlay.dtbo suitable for
> +the domU:
> +
> +    (domU) mkdir -p /sys/kernel/config/device-tree/overlays/new_overlay
> +    (domU) cat overlay_domu.dtbo > 
> /sys/kernel/config/device-tree/overlays/new_overlay/dtbo
> +
> +Finally, if needed, the relevant Linux kernel drive can be probed:
> +
> +    (domU) modprobe module_name.ko
> +
> +Example for domU overlay remove:
> +
> +    (dom0) xl dt-overlay detach overlay.dtbo $domid
> +    (dom0) xl dt-overlay remove overlay.dtbo
I assume we have safety check in place to ensure we can't remove the 
device if it is already attached. Is that correct?
Cheers,

--
Julien Grall