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

[Alex Ghiti]

Le 4/9/21 à 8:07 AM, David Hildenbrand a écrit :
> 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)?


It is in the kernel image and is located in bss which will be in RAM and 
then be backed by a memmap.


> 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?


No I don't create any direct mapping for the text and the rodata.


> [...]
>
> > > 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?