Message-ID: <58ea9e04-d3a8-42e1-9cc3-18c8f141adaf@gmail.com>
Date: Fri, 9 Aug 2024 22:09:09 +0200
From: Klara Modin <klarasmodin@...il.com>
To: Oreoluwa Babatunde <quic_obabatun@...cinc.com>, robh@...nel.org,
 saravanak@...gle.com
Cc: aisheng.dong@....com, hch@....de, m.szyprowski@...sung.com,
 robin.murphy@....com, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, iommu@...ts.linux.dev, will@...nel.org,
 catalin.marinas@....com, kernel@...cinc.com
Subject: Re: [PATCH v7 0/2] Dynamic Allocation of the reserved_mem array

Hi,

On 2024-08-09 20:48, Oreoluwa Babatunde wrote:
> The reserved_mem array is used to store data for the different
> reserved memory regions defined in the DT of a device.  The array
> stores information such as region name, node reference, start-address,
> and size of the different reserved memory regions.
> 
> The array is currently statically allocated with a size of
> MAX_RESERVED_REGIONS(64). This means that any system that specifies a
> number of reserved memory regions greater than MAX_RESERVED_REGIONS(64)
> will not have enough space to store the information for all the regions.
> 
> This can be fixed by making the reserved_mem array a dynamically sized
> array which is allocated using memblock_alloc() based on the exact
> number of reserved memory regions defined in the DT.
> 
> On architectures such as arm64, memblock allocated memory is not
> writable until after the page tables have been setup.
> This is an issue because the current implementation initializes the
> reserved memory regions and stores their information in the array before
> the page tables are setup. Hence, dynamically allocating the
> reserved_mem array and attempting to write information to it at this
> point will fail.
> 
> Therefore, the allocation of the reserved_mem array will need to be done
> after the page tables have been setup, which means that the reserved
> memory regions will also need to wait until after the page tables have
> been setup to be stored in the array.
> 
> When processing the reserved memory regions defined in the DT, these
> regions are marked as reserved by calling memblock_reserve(base, size).
> Where:  base = base address of the reserved region.
> 	size = the size of the reserved memory region.
> 
> Depending on if that region is defined using the "no-map" property,
> memblock_mark_nomap(base, size) is also called.
> 
> The "no-map" property is used to indicate to the operating system that a
> mapping of the specified region must NOT be created. This also means
> that no access (including speculative accesses) is allowed on this
> region of memory except when it is coming from the device driver that
> this region of memory is being reserved for.[1]
> 
> Therefore, it is important to call memblock_reserve() and
> memblock_mark_nomap() on all the reserved memory regions before the
> system sets up the page tables so that the system does not unknowingly
> include any of the no-map reserved memory regions in the memory map.
> 
> There are two ways to define how/where a reserved memory region is
> placed in memory:
> i) Statically-placed reserved memory regions
> i.e. regions defined with a set start address and size using the
>       "reg" property in the DT.
> ii) Dynamically-placed reserved memory regions.
> i.e. regions defined by specifying a range of addresses where they can
>       be placed in memory using the "alloc_ranges" and "size" properties
>       in the DT.
> 
> The dynamically-placed reserved memory regions get assigned a start
> address only at runtime. And this needs to  be done before the page
> tables are setup so that memblock_reserve() and memblock_mark_nomap()
> can be called on the allocated region as explained above.
> Since the dynamically allocated reserved_mem array can only be
> available after the page tables have been setup, the information for
> the dynamically-placed reserved memory regions needs to be stored
> somewhere temporarily until the reserved_mem array is available.
> 
> Therefore, this series makes use of a temporary static array to store
> the information of the dynamically-placed reserved memory regions until
> the reserved_mem array is allocated.
> Once the reserved_mem array is available, the information is copied over
> from the temporary array into the reserved_mem array, and the memory for
> the temporary array is freed back to the system.
> 
> The information for the statically-placed reserved memory regions does
> not need to be stored in a temporary array because their starting
> address is already stored in the devicetree.
> Once the reserved_mem array is allocated, the information for the
> statically-placed reserved memory regions is added to the array.
> 
> Note:
> Because of the use of a temporary array to store the information of the
> dynamically-placed reserved memory regions, there still exists a
> limitation of 64 for this particular kind of reserved memory regions.
>  From my observation, these regions are typically small in number and
> hence I expect this to not be an issue for now.
> 
> Patch Versions:
> v7:
> - Make changes to initialize the reserved memory regions earlier in
>    response to issue reported in v6:
>    https://lore.kernel.org/all/20240610213403.GA1697364@thelio-3990X/
> 
> - For the reserved regions to be setup properly,
>    fdt_init_reserved_mem_node() needs to be called on each of the regions
>    before the page tables are setup. Since the function requires a
>    refernece to the devicetree node of each region, we are not able to
>    use the unflattened_devicetree APIs since they are not available until
>    after the page tables have been setup.
>    Hence, revert the use of the unflatten_device APIs as a result of this
>    limitation which was discovered in v6:
>    https://lore.kernel.org/all/986361f4-f000-4129-8214-39f2fb4a90da@gmail.com/
>    https://lore.kernel.org/all/DU0PR04MB9299C3EC247E1FE2C373440F80DE2@DU0PR04MB9299.eurprd04.prod.outlook.com/
> 
> v6:
> https://lore.kernel.org/all/20240528223650.619532-1-quic_obabatun@quicinc.com/
> - Rebased patchset on top of v6.10-rc1.
> - Addressed comments received in v5 such as:
>    1. Switched to using relevant typed functions such as
>       of_property_read_u32(), of_property_present(), etc.
>    2. Switched to using of_address_to_resource() to read the "reg"
>       property of nodes.
>    3. Renamed functions using "of_*" naming scheme instead of "dt_*".
> 
> v5:
> https://lore.kernel.org/all/20240328211543.191876-1-quic_obabatun@quicinc.com/
> - Rebased changes on top of v6.9-rc1.
> - Addressed minor code comments from v4.
> 
> v4:
> https://lore.kernel.org/all/20240308191204.819487-2-quic_obabatun@quicinc.com/
> - Move fdt_init_reserved_mem() back into the unflatten_device_tree()
>    function.
> - Fix warnings found by Kernel test robot:
>    https://lore.kernel.org/all/202401281219.iIhqs1Si-lkp@intel.com/
>    https://lore.kernel.org/all/202401281304.tsu89Kcm-lkp@intel.com/
>    https://lore.kernel.org/all/202401291128.e7tdNh5x-lkp@intel.com/
> 
> v3:
> https://lore.kernel.org/all/20240126235425.12233-1-quic_obabatun@quicinc.com/
> - Make use of __initdata to delete the temporary static array after
>    dynamically allocating memory for reserved_mem array using memblock.
> - Move call to fdt_init_reserved_mem() out of the
>    unflatten_device_tree() function and into architecture specific setup
>    code.
> - Breaking up the changes for the individual architectures into separate
>    patches.
> 
> v2:
> https://lore.kernel.org/all/20231204041339.9902-1-quic_obabatun@quicinc.com/
> - Extend changes to all other relevant architectures by moving
>    fdt_init_reserved_mem() into the unflatten_device_tree() function.
> - Add code to use unflatten devicetree APIs to process the reserved
>    memory regions.
> 
> v1:
> https://lore.kernel.org/all/20231019184825.9712-1-quic_obabatun@quicinc.com/
> 
> References:
> [1]
> https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/reserved-memory/reserved-memory.yaml#L79
> 
> Oreoluwa Babatunde (2):
>    of: reserved_mem: Restruture how the reserved memory regions are
>      processed
>    of: reserved_mem: Add code to dynamically allocate reserved_mem array
> 
>   drivers/of/fdt.c             |   5 +-
>   drivers/of/of_private.h      |   3 +-
>   drivers/of/of_reserved_mem.c | 231 +++++++++++++++++++++++++++--------
>   3 files changed, 188 insertions(+), 51 deletions(-)
> 

I did not see anything suspicious on my relevant machines with this 
patch series (Raspberry Pi 1 and 3, Edgerouter 6P).

Regards,
Tested-by: Klara Modin <klarasmodin@...il.com>

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