Question about CFG_CORE_HEAP_SIZE

[ydonghyuk]

CFG_CORE_HEAP_SIZE needs to be increased in the ongoing project.
In this case, I would like to inquire about which memory there is a trade-off.

In default settings without any modification,
It is confirmed that 2MB of TZDARM_SIZE is used as TEE_RAM. Is additional allocation being made here?

[jenswi-linaro]

Yes, the heap memory is carved out from that range.

[etienne-lms]

TZDRAM_SIZE covers both TEE_RAM and TA_RAM (in the most common case where CFG_WITH_PAGER=n).
CFG_CORE_HEAP_SIZE indeed that relates to core heap (malloc() and friends) in TEE_RAM only.

TA RAM is set by function core_mmu_get_ta_ram(): it is either hard coded or computed as the unused area of TZDRAM.

• hard coded: defined by TA_RAM_START/TA_RAM_SIZE in platforms' platform_config.h (few platforms)
• computed from TZDRAM_SIZE - TEE_RAM_VA_SIZE (minus TEE_SDP_TEST_MEM_SIZE, likely 0 on most platforms) where TEE_RAM_VA_SIZE is set from CFG_TEE_RAM_VA_SIZE if defined or fallback to CORE_MMU_PGDIR_SIZE. In this case, the bigger CFG_CORE_HEAP_SIZE is, the lower TA_RAM size is. Increasing by few dozen of kBytes should be straight forward. Increasing by many MBytes may need to increase also TZDRAM_SIZE.

[ydonghyuk]

@etienne-lms

You told me that CFG_CORE_HEAP_SIZE is only involved in TEE_RAM, but I don't understand the sentence below.
"the bigger CFG_CORE_HEAP_SIZE is, the lower TA_RAM size is."

Is this only if CFG_TEE_RAM_VA_SIZE is set?

[etienne-lms]

My mistake. Indeed TEE_RAM size is set from CFG_TEE_RAM_VA_SIZE (I agree the name is confusing).
Changing CFG_CORE_HEAP_SIZE will not modify CFG_TEE_RAM_VA_SIZE, hence not modify TA_RAM size.

[ydonghyuk]

@etienne-lms
Thank you for the correction.
If CFG_WITH_PAGER=n and CFG_TEE_RAM_VA_SIZE is not set, I would like to know how TEE_RAM_VA_SIZE is set.