Re: [RFC for-4.8 0/6] xen/arm: Add support for mapping mmio-sram nodes into dom0

[Julien Grall <julien.grall@arm.com>]

Hi Stefano,

On 25/05/16 10:43, Stefano Stabellini wrote:
>> For SRAM it would be normal memory uncached (?) when the property
>> "no-memory-wc" is not present, else TBD.
>>
>> I suspect we would have to relax more MMIOs in the future. Rather than
>> providing a function to map, the code is very similar except the memory
>> attribute, I suggest to provide a list of compatible with the memory attribute
>> to use.
>>
>> All the children node would inherit the memory attribute of the parent.
>>
>> What do you think?
>
> That would work for device tree, but we still need to rely on the
> hypercall for ACPI systems.
>
> Given that it is not easy to add an additional parameter to
> XENMEM_add_to_physmap_range, I think we'll have to provide a new
> hypercall to allow setting attributes other than the Xen default. That
> could be done in Xen 4.8 and Linux >= 4.9.

There is no need to introduce a new hypercall. The 
XENMEM_add_to_physmap_batch contains an unused field ('foreign_id', to 
be renamed) for mapping device MMIOs (see Jan's mail [1]).

XENMEM_add_to_physmap will always map with the default memory attribute 
(Device_nGnRnE) and if the kernel want to use another memory attribute, 
it will have to use XENMEM_add_to_physmap_batch.

With the plan suggested in [2], there are no modifications required in 
Linux for the moment.

Regards,

[1] 
http://lists.xenproject.org/archives/html/xen-devel/2016-05/msg02341.html
[2] 
http://lists.xenproject.org/archives/html/xen-devel/2016-05/msg02347.html

-- 
Julien Grall

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