Date: Mon, 14 Mar 2016 10:34:22 +0900 From: Masanori Ogino <masanori.ogino@...il.com> To: musl@...ts.openwall.com Subject: Re: musl without atomic instructions? 2016-03-13 9:54 GMT+09:00 Masanori Ogino <masanori.ogino@...il.com>: > 2016-03-13 9:21 GMT+09:00 Rich Felker <dalias@...c.org>: >> Minimal profiles for microcontroller use lack it (this was a >> mistake in the riscv ISA specification, IMO), so if supporting these >> ISA levels is interesting, there are at least three options: >> >> 1. Have the kernel trap the unimplemented instructions and emulate >> them. >> >> 2. Have userspace issue a system call to have the kernel mediate >> atomic accesses. >> >> 3. Integrate atomic sequence restart with the scheduler: at scheduling >> time, the kernel determines if the task being resumed was >> interrupted in the middle of a sequence of instructions that's >> supposed to be atomic, and if so, resets the program counter to the >> beginning of the sequence. (This is how pre-v6 ARM and most SH >> models work.) >> >> Option 3 offers by far the best performance but inherently only works >> on uniprocessor. Options 1 and 2 could theoretically support SMP as >> long as the kernel has some other way of ensuring mutual exclusion and >> memory synchronization between the processors. >> >> Of course the best of all worlds is to have the kernel provide a vdso >> function for atomic cas which it can then provide an optimal >> implementation of for the particular processor being used. Then >> baseline-ISA-level riscv binaries would use the vdso, and ones >> targeting an ISA level that's known to have native atomic instructions >> would use the inline instructions. > > OK, I will ask about the current status on the RISC-V sw-dev ML. On sw-dev, Darius Rad taught me that there is a syscall to perform CAS on RISC-V without the A standard extension. CONFIG_RV_SYSRISCV_ATOMIC enables it (with RISC-V patches.) For reference, the source code is here: https://github.com/riscv/riscv-linux/blob/master/arch/riscv/kernel/sys_riscv.c -- Masanori Ogino
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.