Re: [PATCH V2] xen/arm: Increase DOM0_FDT_EXTRA_SIZE to support max reserved memory banks

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 31/03/2026 17:43, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>
> 
> Xen fails to construct the hardware domain's device tree with
> FDT_ERR_NOSPACE (-3) when the host memory map is highly fragmented
> (e.g., numerous reserved memory regions) and the host DTB represents
> RAM compactly (e.g., a single reg pair or just a few).
> 
> This occurs because DOM0_FDT_EXTRA_SIZE underestimates the space
> required for an extra /memory node. While the host DTB might
> represent RAM compactly, make_memory_node() aggregates all
> reserved regions into a single reg property. With NR_MEM_BANKS (256)
> and 64-bit address/size cells, this property can grow up to
> 4KB (256 * 16), easily exceeding the space originally occupied by
> the host DTB's nodes plus the current padding, thereby overflowing
> the allocated buffer.
> 
> Additionally, the SHM regions require space for discrete sub-nodes
> under /reserved-memory node, as well as an appendage to the
> main /memory node. Each of the up to NR_SHMEM_BANKS (32) regions
> triggers the creation of a sub-node with properties (compatible,
> reg, xen,id, and xen,offset). These runtime-generated sub-nodes
> require approximately 142 bytes each, while the appendage consumes
> an additional 16 bytes per region.
> 
> Fix this by increasing DOM0_FDT_EXTRA_SIZE to account for fragmented
> reg properties (NR_MEM_BANKS * 16), the discrete SHM sub-nodes, and
> the SHM appendage to the /memory node (NR_SHMEM_BANKS * (160 + 16)).
> The SHM overhead is conditionally evaluated to avoid over-allocating
> memory when CONFIG_STATIC_SHM=n.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> ---
>   V2:
>    - update commit description
>    - update in-code comment
>    - update macro
> ---
> ---
>  xen/arch/arm/domain_build.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> index 3cd251beed..07f331eac8 100644
> --- a/xen/arch/arm/domain_build.c
> +++ b/xen/arch/arm/domain_build.c
> @@ -98,11 +98,21 @@ int __init parse_arch_dom0_param(const char *s, const 
> char *e)
>  #endif
>  
>  /*
> - * Amount of extra space required to dom0's device tree.  No new nodes
> - * are added (yet) but one terminating reserve map entry (16 bytes) is
> - * added.
> + * Amount of extra space required to dom0's device tree. This covers
> + * nodes generated by Xen, which are not directly copied from the host DTB.
> + * It is calculated as:
> + *  - Space for /hypervisor node (128 bytes).
> + *  - The reserve map terminator (16 bytes).
> + *  - Space for a generated /memory node covering all possible reserved
> + *    memory regions (NR_MEM_BANKS * 16).
> + *  - Space for a generated /reserved-memory node with discrete SHM 
> sub-nodes,
> + *    plus the appendage to the main /memory node (NR_SHMEM_BANKS * (160 + 
> 16)
> + *    bytes). This overhead is dropped when CONFIG_STATIC_SHM is disabled.
>   */
> -#define DOM0_FDT_EXTRA_SIZE (128 + sizeof(struct fdt_reserve_entry))
> +#define DOM0_FDT_EXTRA_SIZE (128 + sizeof(struct fdt_reserve_entry) + \
> +                             (NR_MEM_BANKS * 16) +                    \
NIT: alignment. I'll fix on commit

Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>

~Michal