[PATCH v7] RISC-V: enable XIP
david at redhat.com
Fri Apr 9 13:07:24 BST 2021
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 at konsulko.com>
>>>> 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
>>> 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
@Mike, what's your take on not having a memmap for kernel text and ro data?
David / dhildenb
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