On Tue, Jan 28, 2020 at 05:04:49PM +0100, Florian Weimer wrote: > * Segher Boessenkool: > > >> > I don't think we can save LR in a regular register around the system > >> > call, explicitly in the inline asm statement, because we still have to > >> > generate proper unwinding information using CFI directives, something > >> > that you cannot do from within the asm statement. > > > > Why not? > > As far as I knowm there isn't a CFI directive that allows us to restore > the CFI state at the end of the inline assembly. If we say that LR is > stored in a different register than what the rest of the function uses, > that would lead to incorrect CFI after the exit of the inline assembler > fragment. > > At least that's what I think. Compilers aren't really my thing.
.cfi_restore? Or .cfi_remember_state / .cfi_restore_state, that is probably easiest in inline assembler. > >> > GCC does not model the condition registers, > > > > Huh? It does model the condition register, as 8 registers in GCC's > > internal model (one each for CR0..CR7). > > But GCC doesn't expose them as integers to C code, so you can't do much > without them. Sure, it doesn't expose any other registers directly, either. > >> > We don't have an ELFv2 ABI for 32-bit. I doubt it makes sense to > >> > provide an ELFv1 port for this given that it's POWER9-specific. > > > > We *do* have a 32-bit LE ABI. And ELFv1 is not 32-bit either. Please > > don't confuse these things :-) > > > > The 64-bit LE kernel does not really support 32-bit userland (or BE > > userland), *that* is what you want to say. > > Sorry for the confusion. Is POWER9 running kernels which are not 64-bit > LE really a thing in practice, though? Linux only really supports 64-bit LE userland on p9. Anything else is not supported. > >> > From the glibc perspective, the major question is how we handle run-time > >> > selection of the system call instruction sequence. > > > > Well, if it is inlined you don't have this problem either! :-) > > How so? We would have to put the conditional sequence into all inline > system calls, of course. Ah, if you support older systems in your program as well, gotcha. That is not the usual case (just like people use -mcpu=power9 frequently, which means the resulting program will not run on any older CPU). Segher
