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

On 24-Jul-26 10:32, Padhi, Beleswar wrote:
> 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?
I'm afraid we don't have a solution for that at the moment but this is a
limitation we are aware of. So far, when it comes to /reserved-memory we only
have a static shared memory feature allowing to set up a shmem communication
region between domUs, where the range is exposed to domains in DT following this
spec:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/reserved-memory/xen,shared-memory.txt

Cheers,
Michal

> 
> 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
>>>
>>>