curiosity question about swapcontext libc implementation on x86_64

curiosity question about swapcontext libc implementation on x86_64

[Godmar Back]

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The implementation of the obsolescent makecontext/getcontext/swapcontext
family of functions on x86_64 [1] appears to save a selection of registers.

These include callee-saved regs like rbx, rbp, r12-r15, but also
caller-saved regs like rdi, rsi, rdx, rcx. On the floating point side, it
saves mxcsr, the x87 FPU state via fnstenv, but not the xmm SSE register.
All of these floating point registers are callee-saved based on the AMD64
ABI [2].

It also saves/restores the current signal mask.

Does anyone know why it saves this set of registers/signal mask? Why does
it save callee-saved registers, but not all of them?

 - Godmar


[1]
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86_64/swapcontext.S;h=b2e0f19a9d57e5007b094037b4cf59868029fc36;hb=HEAD
[2]
https://gitlab.com/x86-psABIs/x86-64-ABI/-/jobs/artifacts/master/raw/x86-64-ABI/abi.pdf?job=build

Re: curiosity question about swapcontext libc implementation on x86_64

[Florian Weimer]

* Godmar Back via Libc-help:

> The implementation of the obsolescent makecontext/getcontext/swapcontext
> family of functions on x86_64 [1] appears to save a selection of registers.
>
> These include callee-saved regs like rbx, rbp, r12-r15, but also
> caller-saved regs like rdi, rsi, rdx, rcx. On the floating point side, it
> saves mxcsr, the x87 FPU state via fnstenv, but not the xmm SSE register.
> All of these floating point registers are callee-saved based on the AMD64
> ABI [2].

%rdi, %rsi, %rdx, %rcx are argument registers and need to be restored
for makecontext to work correctly.  As far as I can see, getcontext
wouldn't need to save them, but they need to be restored.

The handling of what is now considered per-thread state (such as
floating point control words or signal masks) is inconsistent.  I
think this is just due to the age of these interfaces, and not
something that was designed explicitly.