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riscv-software-src / riscv-pk

RISC-V Proxy Kernel
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Accessing user space data from machine mode. #259

Closed msingla403 closed 2 years ago

msingla403 commented 2 years ago

I am trying to print a string from machine mode. The string is stored in user mode space.

In user space:

buff : "Hello World\n" &buff (virtual address in user space)= 0x000000000001f480

In machine mode:

addr = Physical address of buff = 0x000000408001f480 printm("%s"\n", addr); leads to exception trap_load_access_fault

PMP entries:

csrw    pmpaddr0, -1
csrw    pmpcfg0, 0x1f

By default in PK, PMP are set to permit access to all of memory.

Does anyone know of the issue? To access user space from Supervisor mode, we need to set SUM bit in mstatus. Are there any similar permission settings that I need to set? Or is there any another way to do this?

Thanks in advance

aswaterman commented 2 years ago

There's nothing like SUM needed in this case. My best guess is that the physical address is incorrect. (0x408001f480 is very large; it suggests you've configured Spike to use more than 512 GiB of memory.)

msingla403 commented 2 years ago

Yes, the physical address is wrong. I used kva2pa to convert the user-space virtual address to the physical address. As the kernel and user-space are present in the same virtual address space, I assumed kva2pa should work.

I will have to use walk on page tables to get the correct physical address. Is there any other way? kva2pa works for kernel-space virtual address as the kernel is present at constant offset.

aswaterman commented 2 years ago

kva2pa only works on kernel virtual addresses. walk is the way to go.

msingla403 commented 2 years ago

I am able to access user space contents. Used walk to retrieve physical address.