Re: [PATCH 1/2] arm64: Reduce add_map() complexity

Pierre-Clément Tosi
2024-03-18T19:59:52Z

Hi Fabio,

On Mon, Mar 18, 2024 at 10:31:25AM -0300, Fabio Estevam wrote:
> Please find the new logs here:
>
> https://pastebin.com/raw/qF3GbJry

I notice that the mem_map in these logs have overlapping ranges, which results
in unnecessary work when creating the PTs. For this reason, it would make sense
to prune it in arch/arm/mach-imx/imx8/cpu.c before calling dcache_enable(), IMO.
On this point, you also have contiguous ranges with identical attributes
(mem_map[0] and mem_map[6]), which could be merged into a single entry. This
could result in more efficient MMU mappings if the mem_map entries can share a
block mapping. Also note that mem_map[4].size=0 so could be dropped.

In any case, if we assume that overlapping mem_map entries are a valid input to
the arch code (maybe not as it leads to potentially ambiguous behavior?), then
41e2787f5ec4 had removed support for splitting existing block mappings.

In your case, my assumption is that the function was then treating block
mappings in the range 0x1c000000-0x80000000 (which get mapped, at least
partially, by mem_map[0], mem_map[1], then mem_map[2]) as table mappings and was
issuing read/write instructions in that range (accessing them as PTEs). As those
seem to be device memory (I see they get mapped as MT_DEVICE_NGNRNE), these
accesses might explain the SError you were experiencing.

Would you mind testing [1] and giving it "Tested-by:" if it addresses the issue?

Thanks,

[1]: https://lore.kernel.org/u-boot/43haokus4jdxguk4arig5tsqcgq2wbezwpbj7oti6pdkvrfual@wa7vz2iypcv5/

--
Pierre