On 22/01/2021 02:27, Nathan Lynch wrote:
Michael Ellerman <m...@ellerman.id.au> writes:
Nathan Lynch <nath...@linux.ibm.com> writes:
Alexey Kardashevskiy <a...@ozlabs.ru> writes:
On 16/01/2021 02:38, Nathan Lynch wrote:
Alexey Kardashevskiy <a...@ozlabs.ru> writes:
On 15/01/2021 09:00, Nathan Lynch wrote:
Memory locations passed as arguments from the OS to RTAS usually need
to be addressable in 32-bit mode and must reside in the Real Mode
Area. On PAPR guests, the RMA starts at logical address 0 and is the
first logical memory block reported in the LPAR’s device tree.
On powerpc targets with RTAS, Linux makes available to user space a
region of memory suitable for arguments to be passed to RTAS via
sys_rtas(). This region (rtas_rmo_buf) is allocated via the memblock
API during boot in order to ensure that it satisfies the requirements
described above.
With radix MMU, the upper limit supplied to the memblock allocation
can exceed the bounds of the first logical memory block, since
ppc64_rma_size is ULONG_MAX and RTAS_INSTANTIATE_MAX is 1GB. (512MB is
a common size of the first memory block according to a small sample of
LPARs I have checked.) This leads to failures when user space invokes
an RTAS function that uses a work area, such as
ibm,configure-connector.
Alter the determination of the upper limit for rtas_rmo_buf's
allocation to consult the device tree directly, ensuring placement
within the RMA regardless of the MMU in use.
Can we tie this with RTAS (which also needs to be in RMA) and simply add
extra 64K in prom_instantiate_rtas() and advertise this address
(ALIGH_UP(rtas-base + rtas-size, PAGE_SIZE)) to the user space? We do
not need this RMO area before that point.
Can you explain more about what advantage that would bring? I'm not
seeing it. It's a more significant change than what I've written
here.
We already allocate space for RTAS and (like RMO) it needs to be in RMA,
and RMO is useless without RTAS. We can reuse RTAS allocation code for
RMO like this:
When you say RMO I assume you are referring to rtas_rmo_buf? (I don't
think it is well-named.)
...
RMO (Real mode offset) is the old term we used to use to refer to what
is now called the RMA (Real mode area). There are still many references
to RMO in Linux, but they almost certainly all refer to what we now call
the RMA.
Yes... but I think in this discussion Alexey was using RMO to stand in
for rtas_rmo_buf, which was what I was trying to clarify.
Correct. Thanks for the clarification, appreciated.
May be store in the FDT as "linux,rmo-base" next to "linux,rtas-base",
for clarity, as sharing symbols between prom and main kernel is a bit
tricky.
The benefit is that we do not do the same thing (== find 64K in RMA)
in 2 different ways and if the RMO allocated my way is broken - we'll
know it much sooner as RTAS itself will break too.
Implementation details aside... I'll grant that combining the
allocations into one in prom_init reduces some duplication in the sense
that both are subject to the same constraints (mostly - the RTAS data
area must not cross a 256MB boundary, while the user region may). But
they really are distinct concerns. The RTAS private data area is
specified in the platform architecture, the OS is obligated to allocate
it and pass it to instantiate-rtas, etc etc. However the user region
(rtas_rmo_buf) is purely a Linux construct which is there to support
sys_rtas.
Now, there are multiple sites in the kernel proper that must allocate
memory suitable for passing to RTAS. Obviously there is value in
consolidating the logic for that purpose in one place, so I'll work on
adding that in v2. OK?
I don't think we want to move any allocations into prom_init.c unless we
have to.
It's best thought of as a trampoline, that runs before the kernel
proper, to transition from live OF to a flat DT environment. One thing
that must be done as part of that is instantiating RTAS, because it's
basically a runtime copy of the live OF. But any other allocs are for
Linux to handle later, IMHO.
Agreed.
Then the only comment I have left is may be use of_address_to_resource()
+ resource_size() instead of of_n_addr_cells()/of_n_size_cells() (like
pseries_memory_block_size()). And now I shut up :) Thanks,
--
Alexey
