Re: [PATCH] xen/dt: Skip non-RAM reserved-memory regions in make_memory_node()

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 30-Oct-26 22:58, Kuan-Wei Chiu wrote:
> Hi Michal,
> 
> On Fri, Oct 30, 2026 at 12:04:58PM +0100, Orzel, Michal wrote:
>>
>>
>> On 28-Oct-26 22:17, Kuan-Wei Chiu wrote:
>>> Xen creates a second /memory node for Dom0 from
>>> bootinfo_get_reserved_mem() without checking whether those regions
>>> belong to host RAM. However, /reserved-memory can also describe
>>> on-chip SRAM or firmware regions outside system RAM. Listing them in a
>>> /memory node causes Dom0 to register non-RAM addresses as system RAM,
>>> conflicting with device MMIO mappings.
>> Hmm, I always thought that /reserved-memory is to exclude regions from memory
>> described under /memory. That's what Xen assumes too. map_range_to_domain() 
>> for
>> example does not call iomem_permit_access() for such regions because we 
>> assume
>> it's RAM. Can you provide more details of your board e.g. the device tree? Is
>> there any documentation I could use to confirm that "reserved-memory can also
>> describe on-chip SRAM or firmware regions outside system RAM"?
>>
> Hmm, your reply initially made me wonder if this was caused by a
> non-compliant downstream device tree on our side, so I checked the
> DT spec and upstream Linux.
> 
> The devicetree spec does not seem to explicitly mention whether
> /reserved-memory regions must fall within the range described by
> /memory. From a quick look at early_init_dt_reserve_memory() in Linux,
> the comment there also mentions not worrying if a no-map region is not
> memory as it will not be mapped.
> 
> In upstream Linux device trees, there are cases like
> arch/mips/boot/dts/mobileye/eyeq5.dtsi and eyeq5-epm5.dts, where
> /memory starts at 0x40000000 and 0x802000000, while /reserved-memory
> defines an on-chip sram region nvram@461fe00 with "nvmem-rmem" outside
> /memory.
> 
> Our board's device tree is not upstream yet, but it follows a similar
> pattern where /reserved-memory defines "nvmem-rmem" and dma pool
> sub-regions inside on-chip sram outside the dram range described by
> /memory.
Thanks for confirmation. I can now see:
https://elixir.bootlin.com/linux/v7.3/source/Documentation/devicetree/bindings/nvmem/rmem.yaml
so indeed /reserved-memory may describe non-RAM regions. I'll get back to your
patch then.

~Michal