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

[Kuan-Wei Chiu <visitorckw@xxxxxxxxx>]

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.

Regards,
Kuan-Wei