Add helper function to enter unprivileged mode #608
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Avoids future unsafe-in-unsafe warning,.
The function pointer is branched to by assembly, so we're relying on a certain ABI.
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https://github.com/thejpster/psp-example/blob/main/src/main.rs contains an example showing how to use it. |
cortex-m/src/psp.rs
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| /// Switch to running on the PSP | ||
| #[cfg(cortex_m)] | ||
| pub fn switch_to_psp<const N: usize>(psp_stack: &Stack<N>, function: extern "C" fn() -> !) -> ! { |
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I wonder if switch_to_psp is the best name, given a) it's a function inside the psp module already so you'll end up with psp::switch_to_psp() and b) it also enters unprivileged mode. Maybe it could be a method on Stack like run() or run_unprivileged() or something? Not hugely worried but it's the only thing that jumped out at me from the PR.
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Oh, right. It seems CONTROL.nPRIV (select Privileged or Unprivileged Mode) and CONTROL.SPSEL (select Main Stack Pointer or Process Stack Pointer) are orthogonal to each other.
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Maybe
Line 197 in a3ff54b
enter_unprivileged_with_psp() and then this function can be psp::enter_unprivileged(), leaving space for some future psp::switch_stack(psp_stack: &Stack).
Now offers both Priv and Unpriv modes, and has a handle to represent ownership of a static Stack object. The load/store check on `Stack::taken` is not perfectly thread safe, but it's probably good enough and doing better requires a critical-section or CAS atomics.
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Updated the demo to match |
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The asm is not Armv6-M compatible but inline(always) hides the issue and allows the code to build. See https://rust.godbolt.org/z/sYWMGah8b and #t-compiler > inline(always) caused inline assembly to not get checked |
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Until I added another function that called the broken function anyway, then it suddently noticed the assembly was wrong |
Turns out we don't check the assembly inside inline(asm) functions until the function is actually called (or referenced).
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Tested with https://github.com/thejpster/psp-example $ cargo run --bin unpriv_demo
Compiling cortex-m v0.7.7 (https://github.com/thejpster/cortex-m?branch=add-unprivileged-mode#ef8164b7)
Compiling cortex-m-rt v0.7.5 (https://github.com/thejpster/cortex-m?branch=add-unprivileged-mode#ef8164b7)
Compiling cortex-m-rt-macros v0.7.5 (https://github.com/thejpster/cortex-m?branch=add-unprivileged-mode#ef8164b7)
Compiling defmt-semihosting v0.3.0
Compiling psp-example v0.1.0 (/home/jonathan/Documents/github/thejpster/psp-example)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.45s
Running `/home/jonathan/Documents/github/thejpster/psp-example/./qemu-run.sh target/thumbv7em-none-eabihf/debug/unpriv_demo`
ELF_BINARY=target/thumbv7em-none-eabihf/debug/unpriv_demo
Running on '-cpu cortex-m4 -machine mps2-an386'...
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[INFO ] Using MSP. addr(x) = 203fffc4 (bin/unpriv_demo.rs:17)
[INFO ] PSP stack is at 20000000..20004000 (bin/unpriv_demo.rs:23)
[INFO ] Got SVCall, ptr=20003fec (bin/unpriv_demo.rs:62)
[INFO ] Got SVCall, ptr=20003ff0 (bin/unpriv_demo.rs:62)
[INFO ] Got SVCall, ptr=00000000 (bin/unpriv_demo.rs:62)
[INFO ] Got exit request, leaving now! (bin/unpriv_demo.rs:64)
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$ cargo run --bin priv_demo
Compiling psp-example v0.1.0 (/home/jonathan/Documents/github/thejpster/psp-example)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s
Running `/home/jonathan/Documents/github/thejpster/psp-example/./qemu-run.sh target/thumbv7em-none-eabihf/debug/priv_demo`
ELF_BINARY=target/thumbv7em-none-eabihf/debug/priv_demo
Running on '-cpu cortex-m4 -machine mps2-an386'...
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[INFO ] Hello! (bin/priv_demo.rs:14)
[INFO ] Using MSP. addr(x) = 203fffc0 (bin/priv_demo.rs:15)
[INFO ] PSP stack is at 20000000..20004000 (bin/priv_demo.rs:20)
[INFO ] User mode, ptr=203fff74 (bin/priv_demo.rs:31)
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Adds a data type for managing a PSP stack, and a function to enter PSP mode using it