PoC: create a syscall by forcing the next CPU instruction #259
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This commit adds a first attempt at adding a HW based syscall
trampoline. Basicallt the FW can set a fixed address in a
register in the API. The app can write an API adress. When
written the HW will (should) force the CPU to read the
instruction pointed to by the address set by the FW.
This probably doesn't work. One problem is probably timing
(cycles between writing the API and loading of the next
instruction). We need to try.
Signed-off-by: Joachim Strömbergson <[email protected]>
We don't want to set which address the CPU should jump to when
a syscall is made, we want to push the jump instruction to
execute. If that works.
Signed-off-by: Joachim Strömbergson <[email protected]>
dehanj
changed the title
dehanj
changed the title
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Description
This PR implements a PoC for a syscall, trap function.
The FW can write a jump instruction (with a target address) into an ADDR_SYSCALL_INSTR API address in tk1. The device app (and FW) can write to ADDR_SYSCALL_START API address in tk1. When written to the CPU will be forced to use the jump address in ADDR_SYSCALL_INSTR as next instruction to execute.
This could, may work. One issue may be timing (as in clock cycles). The ADDR_SYSCALL_START trigger must be set at the right cycle in relation to the CPU reading the next instruction. If it's one cycle too early or too late it will probably crash.
One could also simply force the CPU to use another address (not a specific instruction). But at least during PoC hack, trying to do so made the synthesis tool Yosys to allocate a design 180% of the available resources.
Fixes # (issues)
None
Type of change
Feature PoC
Please tick any that are relevant to this PR and remove any that aren't.
Submission checklist