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

["Padhi, Beleswar" <b-padhi@xxxxxx>]

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