On Mon, 03 Jul 2017 16:06:39 PDT (-0700), [email protected] wrote: > On Thu, Jun 29, 2017 at 02:42:38PM -0700, Palmer Dabbelt wrote: >> On Wed, 28 Jun 2017 15:42:37 PDT (-0700), [email protected] wrote: >> > On Wed, Jun 28, 2017 at 11:55:37AM -0700, Palmer Dabbelt wrote: >> >> diff --git a/arch/riscv/include/uapi/asm/ucontext.h >> >> b/arch/riscv/include/uapi/asm/ucontext.h >> >> new file mode 100644 >> >> index 000000000000..52eff9febcfd >> >> --- /dev/null >> >> +++ b/arch/riscv/include/uapi/asm/ucontext.h >> > ... >> >> +struct ucontext { >> >> + unsigned long uc_flags; >> >> + struct ucontext *uc_link; >> >> + stack_t uc_stack; >> >> + sigset_t uc_sigmask; >> >> + /* glibc uses a 1024-bit sigset_t */ >> >> + __u8 __unused[1024 / 8 - sizeof(sigset_t)]; >> >> + /* last for future expansion */ >> >> + struct sigcontext uc_mcontext; >> >> +}; >> > >> > Any particular reason not to use the asm-generic ucontext? >> >> In the generic ucontext, 'uc_sigmask' is at the end of the structure so it >> can >> be expanded. Since we want our mcontext to be expandable as well, we >> pre-allocate some expandable space for sigmask and then put mcontext at the >> end. >> >> We stole this idea from arm64. > > Curious. __unused seems like overkill to be honest given that expanding > the number of signals up to 128 causes other issues (as discovered on > MIPS e.g. the waitpid() status, with stopsig not fitting below the exit > code (shift 8) and core dump flag (bit 7)), but perhaps it could be > carefully expanded by splitting the stopsig field.
Sorry, I don't understand the intricacies of this in the slightest. In general we try to avoid surprises in software land in RISC-V, so whenever we do something we go look at the most popular architectures (Intel and ARM) and try to ensure we don't paint ourselves into any corners that they didn't. > Looks harmless here I suppose so I defer to others. If it is the > preferred approach does it make sense to make it the "default" for new > architectures at some point? Again, this isn't really my thing, but we chose this because we thought it was the sane way to do it. Unless we're doing something silly, I don't see why it wouldn't be a reasonable default. This is predicated on having expandable architectural state, otherwise putting sigmask at the end seems sane.
