Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone

On Wed, Jul 22, 2020 at 1:23 PM Arnd Bergmann [off-list ref] wrote:

On Wed, Jul 22, 2020 at 9:52 PM Palmer Dabbelt [off-list ref] wrote:

quoted

On Wed, 22 Jul 2020 02:43:50 PDT (-0700), Arnd Bergmann wrote:

quoted

On Tue, Jul 21, 2020 at 9:06 PM Palmer Dabbelt [off-list ref] wrote:
The eventual goal is to have a split of 3840MB for either user or linear map
plus and 256MB for vmalloc, including the kernel. Switching between linear
and user has a noticeable runtime overhead, but it relaxes both the limits
for user memory and lowmem, and it provides a somewhat stronger
address space isolation.

Ya, I think we decided not to do that, at least for now.  I guess the right
answer there will depend on what 32-bit systems look like, and since we don't
have any I'm inclined to just stick to the fast option.

Makes sense. Actually on 32-bit Arm we see fewer large-memory
configurations in new machines than we had in the past before 64-bit
machines were widely available at low cost, so I expect not to see a
lot new hardware with more than 1GB of DDR3 (two 256Mbit x16 chips)
for cost reasons, and rv32 is likely going to be similar, so you may never
really see a need for highmem or the above hack to increase the
size of the linear mapping.

I just noticed that rv32 allows 2GB of lowmem rather than just the usual
768MB or 1GB, at the expense of addressable user memory. This seems
like an unusual choice, but I also don't see any reason to change this
or make it more flexible unless actual users appear.

I am a bit confused here. As per my understanding, RV32 supports 1GB
of lowmem only
as the page offset is set to 0xC0000000. The config option
MAXPHYSMEM_2GB is misleading
as RV32 actually allows 1GB of physical memory only. Any memory blocks beyond
DRAM + 1GB are removed in setup_bootmem. IMHO, The current config
should clarify that.

Moreover, we should add 2G split under a separate configuration if we
want to support that.

       Arnd

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-- 
Regards,
Atish