Re: [PATCH V2] xen/dom0less: Calculate guest DTB size based on MAX_VIRT_CPUS

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

Hello Oleksandr,

On 12/3/25 7:58 PM, Oleksandr Tyshchenko wrote:
> Creating a dom0less guest with a high vCPU count (e.g., >32) fails
> because the fixed 4KiB device tree buffer (DOMU_DTB_SIZE) overflows
> during creation.
>
> The FDT nodes for each vCPU are the primary consumer of space,
> and the previous fixed-size buffer was insufficient.
>
> This patch replaces the fixed size with a formula that calculates
> the required buffer size based on a fixed baseline plus a scalable
> portion for each potential vCPU up to the MAX_VIRT_CPUS limit.
>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> ---
> V1: 
> https://patchew.org/Xen/20251202193246.3357821-1-oleksandr._5Ftyshchenko@xxxxxxxx/
>
>    V2:
>     - update commit subj/desc
>     - use a formula that accounts MAX_VIRT_CPUS
>     - add BUILD_BUG_ON
> ---
> ---
>   xen/common/device-tree/dom0less-build.c | 16 +++++++++++++---
>   1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/xen/common/device-tree/dom0less-build.c 
> b/xen/common/device-tree/dom0less-build.c
> index 3f5b987ed8..38a5830813 100644
> --- a/xen/common/device-tree/dom0less-build.c
> +++ b/xen/common/device-tree/dom0less-build.c
> @@ -461,15 +461,25 @@ static int __init domain_handle_dtb_boot_module(struct 
> domain *d,
>   
>   /*
>    * The max size for DT is 2MB. However, the generated DT is small (not 
> including
> - * domU passthrough DT nodes whose size we account separately), 4KB are enough
> - * for now, but we might have to increase it in the future.
> + * domU passthrough DT nodes whose size we account separately). The size is
> + * calculated from a fixed baseline plus a scalable portion for each potential
> + * vCPU node up to the system limit (MAX_VIRT_CPUS), as the vCPU nodes are
> + * the primary consumer of space.
> + *
> + * The baseline of 2KiB is a safe buffer for all non-vCPU FDT content.
> + * Empirical testing with the maximum number of other device tree nodes shows
> + * a final compacted base size of ~1.5KiB. The 128 bytes per vCPU is derived
> + * from a worst-case analysis of the FDT construction-time size for a single
> + * vCPU node.
>    */
> -#define DOMU_DTB_SIZE 4096
> +#define DOMU_DTB_SIZE (2048 + (MAX_VIRT_CPUS * 128))
>   static int __init prepare_dtb_domU(struct domain *d, struct kernel_info 
> *kinfo)
>   {
>       int addrcells, sizecells;
>       int ret, fdt_size = DOMU_DTB_SIZE;
>   
> +    BUILD_BUG_ON(DOMU_DTB_SIZE > SZ_2M);
> +
>       kinfo->phandle_intc = GUEST_PHANDLE_GIC;
>   
>   #ifdef CONFIG_GRANT_TABLE
LGTM:
 Reviewed-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Thanks!

~ Oleksii