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

[Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>]

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)
 static int __init prepare_dtb_domU(struct domain *d, struct kernel_info *kinfo)
 {
     int addrcells, sizecells;
-- 
2.34.1