Re: [Query] Unable to probe reserved-memory carveouts in DomU with Xen

Hello,

Thanks for the response! I see, this would solve the cma problem. But 
there are usecases
when we want to use the reserved-memory node; one example is the 
firmware for
remote processors. In that case, how do we passthrough reserved-memory 
nodes to
DomUs?

Example:
  60
  61                 mcu_r52_0_fw_memory_region: memory@a1000000 {
  62                         compatible = "shared-dma-pool";
  63                         reg = <0x00 0xa1000000 0x00 0x100000>;
  64                         no-map;
  65                 };

Thanks,
Beleswar

On 7/24/2026 1:52 PM, Orzel, Michal wrote:
> Hi,
>
> I'm afraid this will not work. The reason is that Xen only copies nodes from the
> passthrough DTB to final DTB that are placed under `/passthrough` node and
> `/aliases`. /reserved-memory needs to be under root node `/`, therefore it will
> not be copied. However, it seems like you are not using CMA node specifying a
> fixed memory region (using `reg`) but size. In that case you should be able to
> achieve the same using `cma=128M` kernel cmdline parameter.
>
> ~Michal
>
> On 24-Jul-26 08:55, Beleswar Prasad Padhi wrote:
>> Hi all,
>>
>> I have been trying to use reserved-memory carveouts in DomU, but failing constantly.
>>
>> This is the log from Dom1 boot:
>>
>> [    0.000000] earlycon: uart8250 at MMIO32 0x0000000053914000 (options '115200n
>> 8')
>> [    0.000000] printk: legacy bootconsole [uart8250] enabled
>> [    0.000000] Xen 4.21 support found
>> [    0.000000] efi: UEFI not found.
>> *[    0.000000] OF: reserved mem: Reserved memory: No reserved-memory node in the DT*
>> [    0.000000] Zone ranges:
>> [    0.000000]   DMA      [mem 0x0000000040000000-0x0000000051ffffff]
>> [    0.000000]   DMA32    empty
>> [    0.000000]   Normal   empty
>> [    0.000000] Movable zone start for each node
>> [    0.000000] Early memory node ranges
>> [    0.000000]   node   0: [mem 0x0000000040000000-0x0000000051ffffff]
>> [    0.000000] Initmem setup node 0 [mem 0x0000000040000000-0x0000000051ffffff]
>> [    0.000000] On node 0, zone DMA: 24576 pages in unavailable ranges
>> [    0.000000] cma: Reserved 32 MiB at 0x000000004f800000
>>
>> This is my passthrough.dts:
>>
>>      1 /*
>>      2 * This is a partial DT for passthrough node to DomU.
>>      3 */
>>      4 #include <dt-bindings/interrupt-controller/irq.h>
>>      5 #include <dt-bindings/interrupt-controller/arm-gic.h>
>>      6
>>      7 /dts-v1/;
>>      8
>>      9 / {
>>     10         #address-cells = <2>;
>>     11         #size-cells = <1>;
>>     12
>> ~  13         reserved-memory {
>> ~  14                 #address-cells = <2>;
>> ~  15                 #size-cells = <2>;
>> +  16                 ranges;
>> +  17
>> +  18                 /* CMA region for framebuffer allocation */
>> +  19                 linux,cma {
>> +  20                         compatible = "shared-dma-pool";
>> +  21                         reusable;
>> +  22                         size = <0x00 0x08000000>; /* 128MB CMA pool */
>> +  23                         linux,cma-default;
>> +  24                 };
>>     25         };
>>     26
>>     27         passthrough {
>>     28                 compatible = "simple-bus";
>>     29                 ranges;
>>     30                 #address-cells = <2>;
>>     31                 #size-cells = <1>;
>>     32
>>     33                 misc_clk: misc_clk {
>>     34                         #clock-cells = <0>;
>>     35                         clock-frequency = <100000000>;
>>     36                         compatible = "fixed-clock";
>>     37                 };
>>
>>     73                 uart@53914000 {
>>     74                         compatible = "ns16550a";
>>     75                         reg = <0x00 0x53914000 0x1000>;
>> ~_ 76                         interrupts = <0 302 4>;
>>     77                         reg-shift = <2>;
>>     78                         reg-io-width = <4>;
>>     79                         clocks = <&misc_clk>;
>>     80                         oversampling-rate = <16>;
>>     81                         programmable-char-timeout = <4>;
>>     82                         status = "okay";
>>     83                         xen,path = "/bus@100000/bus@22000000/uart@53914000";
>>     84                         xen,reg = <0x0 0x53914000 0x1000 0x0 0x53914000>;
>>     85                         xen,force-assign-without-iommu;
>>     86                 };
>>     87          };
>>
>>     88    };
>>
>>
>> And this is my dom1.cfg file:
>>
>>   1 name = "dom1"
>>   2 memory = 256
>>   3 vcpus = 4
>>   4
>>   5 kernel = "/boot/xen-dom1/Image"
>>   6 extra = "console=ttyS0,115200n8 earlycon=uart8250,mmio32,0x53914000,115200n8 loglevel=8 root=/dev/xvda rw"
>>   7
>>   8 # UART1 (0x53914000) passthrough to Dom1 console
>>   9 iomem = ["0x53914,1"]
>> 11 irqs = [ 334 ]
>> 12 dtdev = [ "/bus@100000/bus@22000000/uart@53914000"]
>> 13 vif = ["script=vif-route"]
>> 14 device_tree = "/boot/xen-dom1/passthrough.dtb"
>> 15 #
>> 17 disk = ["phy:/dev/vda2,xvda,rw"]
>>
>>
>> Am I doing something wrong? Any help or debugging leads would be really appreciated...
>>
>> Thank you,
>> Beleswar
>>
>>