Re: [REBASE PATCH v5 08/17] arm64: mm: Add dynamic ramoops region support through command line

[Pavan Kondeti]

On Thu, Oct 05, 2023 at 04:52:20PM +0530, Mukesh Ojha wrote:
> Sorry for the late reply, was on a long vacation.
> 
> On 9/14/2023 4:47 AM, Kees Cook wrote:
> > On Tue, Sep 12, 2023 at 11:18:20AM +0100, Will Deacon wrote:
> > > On Mon, Sep 11, 2023 at 04:23:50PM +0530, Mukesh Ojha wrote:
> > > > The reserved memory region for ramoops is assumed to be at a fixed
> > > > and known location when read from the devicetree. This may not be
> > > > required for something like Qualcomm's minidump which is interested
> > > > in knowing addresses of ramoops region but it does not put hard
> > > > requirement of address being fixed as most of it's SoC does not
> > > > support warm reset and does not use pstorefs at all instead it has
> > > > firmware way of collecting ramoops region if it gets to know the
> > > > address and register it with apss minidump table which is sitting
> > > > in shared memory region in DDR and firmware will have access to
> > > > these table during reset and collects it on crash of SoC.
> > > > 
> > > > So, add the support of reserving ramoops region to be dynamically
> > > > allocated early during boot if it is request through command line
> > > > via 'dyn_ramoops_size=' and fill up reserved resource structure and
> > > > export the structure, so that it can be read by ramoops driver.
> > > > 
> > > > Signed-off-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
> > > > ---
> > > >   arch/arm64/mm/init.c       | 94 ++++++++++++++++++++++++++++++++++++++++++++++
> > > 
> > > Why does this need to be in the arch code? There's absolutely nothing
> > > arm64-specific here.
> > 
> > I would agree: this needs to be in ramoops itself, IMO. It should be a
> > ramoops module argument, too.
> > 
> > It being unhelpful for systems that don't have an external consumer is
> > certainly true, but I think it would still make more sense for this
> > change to live entirely within ramoops. Specifically: you're
> > implementing a pstore backend behavioral change. In the same way that
> > patch 10 is putting the "output" side of this into pstore/, I'd expect
> > the "input" side also in pstore/
> 
> How do we reserve memory? are you suggesting to use dma api's for
> dynamic ramoops ?
> 
Sharing my thoughts:

Your patch is inspired from how kexec allocate memory for crash kernel
right? There is a series [1] which moved arch code (ARM64/x86) to
generic kexec core. Something we should also do as the feedback
received here.

Coming to how part, we still have to use memblock API to increase the chance
of allocating contiguous memory. Since PSTORE_RAM can also be
compiled as a module, we probably need another pstore layer that needs to
be built statically in kernel to allocate memory using memblock API.
once slab is available, all memblock API will re-direct to slab
allocations. This layer can be enabled via ARCH_WANTS_PSTORE_xxx or 
another config that only supports 'y'. PSTORE_RAM can still be a module but 
when this layer is available, it supports dynamic ramoops. Another option 
would be just including this layer in PSTORE RAM module but take away module 
option  when this layer is enabled.


[1]
https://lore.kernel.org/all/20211020020317.1220-6-thunder.leizhen@xxxxxxxxxx/