Re: [PATCH] xen/dom0less: Increase guest DTB size for high-vCPU guests

[Grygorii Strashko <grygorii_strashko@xxxxxxxx>]

On 02.12.25 21:32, Oleksandr Tyshchenko wrote:
> Creating a guest with a high vCPU count (e.g., >32) fails because
> the guest's device tree buffer (DOMU_DTB_SIZE) overflows during creation.
> The FDT nodes for each vCPU quickly exhaust the 4KiB buffer,
> causing a guest creation failure.
>
> Increase the buffer size to 16KiB to support guests up to
> the MAX_VIRT_CPUS limit (128).
>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> ---
> Noticed when testing the boundary conditions for dom0less guest
> creation on Arm64.
>
> Domain configuration:
> fdt mknod /chosen domU0
> fdt set /chosen/domU0 compatible "xen,domain"
> fdt set /chosen/domU0 \#address-cells <0x2>
> fdt set /chosen/domU0 \#size-cells <0x2>
> fdt set /chosen/domU0 memory <0x0 0x10000 >
> fdt set /chosen/domU0 cpus <33>
> fdt set /chosen/domU0 vpl011
> fdt mknod /chosen/domU0 module@40400000
> fdt set /chosen/domU0/module@40400000 compatible  "multiboot,kernel" 
> "multiboot,module"
> fdt set /chosen/domU0/module@40400000 reg <0x0 0x40400000 0x0 0x16000 >
> fdt set /chosen/domU0/module@40400000 bootargs "console=ttyAMA0"
>
> Failure log:
> (XEN) Xen dom0less mode detected
> (XEN) *** LOADING DOMU cpus=33 memory=0x10000KB ***
> (XEN) Loading d1 kernel from boot module @ 0000000040400000
> (XEN) Allocating mappings totalling 64MB for d1:
> (XEN) d1 BANK[0] 0x00000040000000-0x00000044000000 (64MB)
> (XEN) Device tree generation failed (-22).
> (XEN)
> (XEN) ****************************************
> (XEN) Panic on CPU 0:
> (XEN) Could not set up domain domU0 (rc = -22)
> (XEN) ****************************************
> ---
> ---
>   xen/common/device-tree/dom0less-build.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/xen/common/device-tree/dom0less-build.c 
> b/xen/common/device-tree/dom0less-build.c
> index 3f5b987ed8..d7d0a47b97 100644
> --- a/xen/common/device-tree/dom0less-build.c
> +++ b/xen/common/device-tree/dom0less-build.c
> @@ -461,10 +461,12 @@ 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
> + * primarily driven by the number of vCPU nodes. The previous 4KiB buffer was
> + * insufficient for guests with high vCPU counts, so it has been increased
> + * to support up to the MAX_VIRT_CPUS limit (128).
>    */
> -#define DOMU_DTB_SIZE 4096
> +#define DOMU_DTB_SIZE (4096 * 4)
May be It wants Kconfig?
Or some formula which accounts MAX_VIRT_CPUS?

>   static int __init prepare_dtb_domU(struct domain *d, struct kernel_info 
> *kinfo)
>   {
>       int addrcells, sizecells;
--
Best regards,
-grygorii