----- On Jun 13, 2018, at 7:48 AM, heiko carstens heiko.carst...@de.ibm.com wrote:
> On Mon, Jun 11, 2018 at 03:49:18PM -0400, Mathieu Desnoyers wrote: >> Hi! >> >> Good news! The restartable sequences (rseq) system call is now merged into >> the >> master >> branch of the Linux kernel within the 4.18 merge window: >> >> https://github.com/torvalds/linux/commit/d82991a8688ad128b46db1b42d5d84396487a508 >> >> It would be important to discuss how we should proceed to integrate the >> library >> part >> of rseq (see tools/testing/selftests/rseq/rseq*.{ch}) into glibc, or if it >> should >> live in a standalone project. > > Is there any documentation available of what is the exact semantics of the > functions that have to be implemented for additional architectures? It's documented on top of kernel/rseq.c: /* * * Restartable sequences are a lightweight interface that allows * user-level code to be executed atomically relative to scheduler * preemption and signal delivery. Typically used for implementing * per-cpu operations. * * It allows user-space to perform update operations on per-cpu data * without requiring heavy-weight atomic operations. * * Detailed algorithm of rseq user-space assembly sequences: * * init(rseq_cs) * cpu = TLS->rseq::cpu_id_start * [1] TLS->rseq::rseq_cs = rseq_cs * [start_ip] ---------------------------- * [2] if (cpu != TLS->rseq::cpu_id) * goto abort_ip; * [3] <last_instruction_in_cs> * [post_commit_ip] ---------------------------- * * The address of jump target abort_ip must be outside the critical * region, i.e.: * * [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip] * * Steps [2]-[3] (inclusive) need to be a sequence of instructions in * userspace that can handle being interrupted between any of those * instructions, and then resumed to the abort_ip. * * 1. Userspace stores the address of the struct rseq_cs assembly * block descriptor into the rseq_cs field of the registered * struct rseq TLS area. This update is performed through a single * store within the inline assembly instruction sequence. * [start_ip] * * 2. Userspace tests to check whether the current cpu_id field match * the cpu number loaded before start_ip, branching to abort_ip * in case of a mismatch. * * If the sequence is preempted or interrupted by a signal * at or after start_ip and before post_commit_ip, then the kernel * clears TLS->__rseq_abi::rseq_cs, and sets the user-space return * ip to abort_ip before returning to user-space, so the preempted * execution resumes at abort_ip. * * 3. Userspace critical section final instruction before * post_commit_ip is the commit. The critical section is * self-terminating. * [post_commit_ip] * * 4. <success> * * On failure at [2], or if interrupted by preempt or signal delivery * between [1] and [3]: * * [abort_ip] * F1. <failure> */ > I could look at rseq-skip.h and e.g. rseq-x86.h and try to figure out what > would be the correct implementation for s390. But having that somewhere > written down, e.g. as comments in one of the implementations, would be very > helpful. The first architecture implemented was rseq-x86.h. Boqun derived rseq-ppc.h from it, and I derived rseq-arm.h from it. Feel free to start from whichever architecture has the instruction set which is most similar to yours. Thanks! Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com