Message-ID: <4b6295c6-d792-4fc8-893c-3be0b26cd22c@gmail.com>
Date: Sat, 31 Aug 2024 13:36:28 +0200
From: Klara Modin <klarasmodin@...il.com>
To: Oreoluwa Babatunde <quic_obabatun@...cinc.com>, robh@...nel.org
Cc: andy@...ck.fi.intel.com, aisheng.dong@....com, catalin.marinas@....com,
 devicetree@...r.kernel.org, hch@....de, iommu@...ts.linux.dev,
 kernel@...cinc.com, linux-kernel@...r.kernel.org, m.szyprowski@...sung.com,
 robin.murphy@....com, saravanak@...gle.com, will@...nel.org
Subject: Re: [PATCH v8 0/2] Dynamic Allocation of the reserved_mem array

Hi,

On 2024-08-30 18:28, 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.
> 

I tested the series on the same systems as previously and did not see 
anything suspicious.

In addition I tested my x86_64 desktop with CONFIG_OF enabled and this 
new version boots fine for me and did not show any new issues from what 
I could tell.

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

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