[U-Boot] [RFC PATCH] armv8: Extend modification of MMU tables

[york sun]

On 02/10/2017 09:51 AM, york.sun at nxp.com wrote:
> On 02/10/2017 09:34 AM, Stephen Warren wrote:
>>
>> Still, I looked at all the "struct mm_region" tables in U-Boot and
>> couldn't find any that set PXN/UXN for normal memory, and I don't
>> believe this function will be called for anything other than normal
>> memory. As such, this change should be fine in practice. However, I'm
>> still a bit hesitant about this change. Can updating PXN/UXN be made
>> optional? Related: Why would we need to change PXN/UXN at run-time;
>> shouldn't the base struct mm_region table be set up correctly from the
>> start?
>

I forgot to add here. If the DDR mapping is created _before_ DDR is 
initialized, it has to be device memory, not normal memory to avoid 
speculative access. I got from ARM support that the PXN/UXN must be set 
for device memory, otherwise it is architecturally undefined. I would 
like to skipping the mapping for DDR to begin with and add it back 
later. However, current code counts the highest address in predefined 
mapping and decides the TCR. So I have to have something for my highest 
mapping to hold the place. The only safe way is to map it as device 
memory with PXN/UXN to avoid hitting the speculative access issue.

Now you see why I have to update the PXN/UXN bits.

One more point, I have to avoid using the backup MMU table as current 
mmu_set_region_dcache_behaviour() does. For my early MMU table (long 
story why I have to use MMU early, in short, for performance on 
emulators), the space is very limited and I cannot afford extra space. I 
will follow ARM's suggestion to do break-before-make to update MMU.

York