Re: [PATCH v7] RISC-V: enable XIP

[David Hildenbrand]

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