On 09.04.21 13:39, Alex Ghiti wrote:
Hi David,
Le 4/9/21 à 4:23 AM, David Hildenbrand a écrit :
On 09.04.21 09:14, Alex Ghiti wrote:
Le 4/9/21 à 2:51 AM, Alexandre Ghiti a écrit :
From: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
Introduce XIP (eXecute In Place) support for RISC-V platforms.
It allows code to be executed directly from non-volatile storage
directly addressable by the CPU, such as QSPI NOR flash which can
be found on many RISC-V platforms. This makes way for significant
optimization of RAM footprint. The XIP kernel is not compressed
since it has to run directly from flash, so it will occupy more
space on the non-volatile storage. The physical flash address used
to link the kernel object files and for storing it has to be known
at compile time and is represented by a Kconfig option.
XIP on RISC-V will for the time being only work on MMU-enabled
kernels.
I added linux-mm and linux-arch to get feedbacks because I noticed that
DEBUG_VM_PGTABLE fails for SPARSEMEM (it works for FLATMEM but I think
it does not do what is expected): the fact that we don't have any struct
page to back the text and rodata in flash is the problem but to which
extent ?
Just wondering, why can't we create a memmap for that memory -- or is it
even desireable to not do that explicity? There might be some nasty side
effects when not having a memmap for text and rodata.
Do you have examples of such effects ? Any feature that will not work
without that ?
At least if it's not part of /proc/iomem in any way (maybe "System RAM"
is not what we want without a memmap, TBD), kexec-tools won't be able to
handle it properly e.g., for kdump. But not sure if that is really
relevant in your setup.
Regarding other features, anything that does a pfn_valid(),
pfn_to_page() or pfn_to_online_page() would behave differently now --
assuming the kernel doesn't fall into a section with other System RAM
(whereby we would still allocate the memmap for the whole section).
I guess you might stumble over some surprises in some code paths, but
nothing really comes to mind. Not sure if your zeropage is part of the
kernel image on RISC-V (I remember that we sometimes need a memmap
there, but I might be wrong)?
I assume you still somehow create the direct mapping for the kernel,
right? So it's really some memory region with a direct mapping but
without a memmap (and right now, without a resource), correct?
[...]
Also, will that memory properly be exposed in the resource tree as
System RAM (e.g., /proc/iomem) ? Otherwise some things (/proc/kcore)
won't work as expected - the kernel won't be included in a dump.
I have just checked and it does not appear in /proc/iomem.
Ok your conclusion would be to have struct page, I'm going to implement
this version then using memblock as you described.
Let's first evaluate what the harm could be. You could (and should?)
create the kernel resource manually - IIRC, that's independent of the
memmap/memblock thing.
@Mike, what's your take on not having a memmap for kernel text and ro data?
--
Thanks,
David / dhildenb
