Re: [PATCH v5 5/7] xen/arm: Add XEN_DOMCTL_dt_overlay and device attachment to domains

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 24/05/2024 03:18, Stefano Stabellini wrote:
> From: Henry Wang <xin.wang2@xxxxxxx>
>
> In order to support the dynamic dtbo device assignment to a running
> VM, the add/remove of the DT overlay and the attach/detach of the
> device from the DT overlay should happen separately. Therefore,
> repurpose the existing XEN_SYSCTL_dt_overlay to only add the DT
> overlay to Xen device tree, instead of assigning the device to the
> hardware domain at the same time. It is OK to change the sysctl behavior
> as this feature is experimental so changing sysctl behavior and breaking
> compatibility is OK.
>
> Add the XEN_DOMCTL_dt_overlay with operations
> XEN_DOMCTL_DT_OVERLAY_ATTACH to do the device assignment to the domain.
>
> The hypervisor firstly checks the DT overlay passed from the toolstack
> is valid. Then the device nodes are retrieved from the overlay tracker
> based on the DT overlay. The attach of the device is implemented by
> mapping the IRQ and IOMMU resources. All devices in the overlay are
> assigned to a single domain.
>
> Also take the opportunity to make one coding style fix in sysctl.h.
>
> xen,reg is to be used to handle non-1:1 mappings but it is currently
> unsupported.
This means that we would still try to use 1:1 mappings for non-directmap 
domain. Given that the overlay is a blob, I am a bit concerned that the 
user may not notice any clash and it would be difficult to debug.
Therefore, I would like xen,reg to be mandatory when using non 
directmapped domain. For now, the best approach would be to prevent 
device assignment if !is_domain_direct_mapped().
> +long dt_overlay_domctl(struct domain *d, struct xen_domctl_dt_overlay *op)
> +{
> +    long ret;
> +    void *overlay_fdt;
> +
> +    if ( op->overlay_op != XEN_DOMCTL_DT_OVERLAY_ATTACH )
> +        return -EOPNOTSUPP;
> +
> +    if ( op->overlay_fdt_size == 0 || op->overlay_fdt_size > KB(500) )
Please add #define DT_OVERLAY_MAX_SIZE KB(500) and use it here and the 
other place.
Cheers,

--
Julien Grall