On 2020-10-30 01:38, Mike Rapoport wrote:
On Thu, Oct 29, 2020 at 02:29:27PM -0700, Sudarshan Rajagopalan wrote:
Hello all,
We have a usecase where a module driver adds certain memory blocks
using
add_memory_driver_managed(), so that it can perform memory hotplug
operations on these blocks. In general, these memory blocks aren’t
something
that gets physically added later, but is part of actual RAM that
system
booted up with. Meaning – we set the ‘mem=’ cmdline parameter to limit
the
memory and later add the remaining ones using add_memory*() variants.
The basic idea is to have driver have ownership and manage certain
memory
blocks for hotplug operations.
For the driver be able to know how much memory was limited and how
much
actually present, we take the delta of ‘bootmem physical end address’
and
‘memblock_end_of_DRAM’. The 'bootmem physical end address' is obtained
by
scanning the reg values in ‘memory’ DT node and determining the max
{addr,size}. Since our driver is getting modularized, we won’t have
access
to memblock_end_of_DRAM (i.e. end address of all memory blocks after
‘mem=’
is applied).
So checking if memblock_{start/end}_of_DRAM() symbols can be exported?
Also,
this information can be obtained by userspace by doing ‘cat
/proc/iomem’ and
greping for ‘System RAM’. So wondering if userspace can have access to
such
info, can we allow kernel module drivers have access by exporting
memblock_{start/end}_of_DRAM().
These functions cannot be exported not because we want to hide this
information from the modules but because it is unsafe to use them.
On most architecturs these functions are __init so they are discarded
after boot anyway. Beisdes, the memory configuration known to memblock
might be not accurate in many cases as David explained in his reply.
I don't see how information contained in memblock_{start/end}_of_DRAM()
is considered hidden if the information can be obtained using 'cat
/proc/iomem'. The memory resource manager adds these blocks either in
"System RAM", "reserved", "Kernel data/code" etc. Inspecting this, one
could determine whats the start and end of memblocks.
I agree on the part that its __init annotated and could be removed after
boot. This is something that the driver can be vary of too.
Or are there any other ways where a module driver can get the end
address of
system memory block?
What do you mean by "system memory block"? There could be a lot of
interpretations if you take into account memory hotplug, "mem=" option,
reserved and firmware memory.
I meant the physical end address of memblock. The equivalent of
memblock_end_of_DRAM.
I'd suggest you to describe the entire use case in more detail. Having
the complete picture would help finding a proper solution.
The usecase in general is have a way to add/remove and online/offline
certain memory blocks which are part of boot. We do this by limiting the
memory using "mem=" and latter add the remaining blocks using
add_memory_driver_mamanaged().
Sudarshan
--
Sincerely yours,
Mike.
Sudarshan
--
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