ARM eglibc: adding ucontext?

[Peter Maydell]

On 24 October 2011 20:05, Michael K. Edwards <m.k.edwards at gmail.com> wrote:
> The calling convention for makecontext() isn't just an issue for standards
> lawyers, it's an actual problem on ARM, especially on hard-float systems.
> According to the standard, "The application shall ensure that the value of
> argc matches the number of arguments of type int passed to func; otherwise,
> the behavior is undefined."  If you read this to imply that func() must be a
> non-variadic function that takes zero or more int arguments, and call it as
> if it took exactly as many int arguments as fit in registers -- stuffing
> extras onto the stack if necessary -- then you're OK.  But supporting more
> general calling conventions for func() gets hairy, and correctly supporting
> both variadic and non-variadic functions on a hard-float system is
> impossible.

FWIW, the function QEMU uses is:
static void coroutine_trampoline(int i0, int i1), so we are not trying
to take advantage of the hairier cases.

> Note that you will need to save/restore FP state in all swapcontext()
> calls.  User thread context switch code doesn't have access to the FPEXC
> register and thus can't identify non-FP-using contexts and skip the q4-q7
> save/restore.  However, the standard is silent on whether you can have
> context-specific settings of floating point exception control, rounding
> mode, etc.  If QEMU's coroutines need this, you'll need to save/restore
> FPSCR as well.

As it happens QEMU coroutines use swapcontext() only as a way to initially
switch to a newly allocated stack -- actual switching between coroutines
is done via setjmp()/longjmp(). (On reflection this does seem a bit odd...)

(the actual code is here:
http://git.linaro.org/gitweb?p=people/pmaydell/qemu-arm.git;a=blob;f=coroutine-ucontext.c;h=2b8d3e9c128dccc551a3ae27f7ffd50b343b11cc;hb=master
)

-- PMM