Trying to remove mmu for riscv32 bao and need help

[yangn0]

Hi,

I'm trying to remove the mmu of riscv32. I plan to use standard PMP to implement isolation and vplic with a modified opensbi.

But I encountered a problem:
In file boot.S, line 152.

The s6 is the size of VM. Why _page_tables_start and _page_tables_end need to add s6 size?

[josecm]

@yangn0 Those are very nice news! In the past we have discussed doing exactly that.

Answering your question directly. You can see we set s6 with extra_allocated_phys_mem which is a symbol defined in the linker script describing the size the VM images take when embedding them in the hypervisor image directly. This means there is a gap between where .bss starts and the actual end of the hypervisor image in physical space. Because we don't want to have this gap on the virtual address space, we rewind the location counter to "remove" this from the virtual image.

Now, the la pseudoisntruction works with relative addressing. So it will give the relative address of the page tables taking into account the offset of the virtual addresses. Because this gap is effectively removed from the virtual address space and, at that point in the code we are working with physical addresses (before enabling the mmu) we need to add that offset by hand.

A bit confusing I know. But I hope it is sufficient for you to get the point.

However, let me point out that we do have an mpu memory protection profile that would fit using the PMP. If setup your port to use that infrastructure, most of the code is already ready for running without an mmu. Of course you'll need to adapt everything else so that bao runs on machine mode.

Let me know of your progess! This is really interesting!

[yangn0]

Thank you very much! I understand now.

an mpu memory protection profile that would fit using the PMP.
Could you tell me where to get this profile?

And

Why add PT_SIZE here?