----- On Nov 23, 2017, at 5:51 PM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote: > On Thu, 23 Nov 2017, Mathieu Desnoyers wrote: >> ----- On Nov 22, 2017, at 2:37 PM, Will Deacon will.deacon@xxxxxxx wrote: >> > On Wed, Nov 22, 2017 at 08:32:19PM +0100, Peter Zijlstra wrote: >> >> >> >> So what exactly is the problem of leaving out the whole cpu_opv thing >> >> for now? Pure rseq is usable -- albeit a bit cumbersome without >> >> additional debugger support. >> > >> > Drive-by "ack" to that. I'd really like a working rseq implementation in >> > mainline, but I don't much care for another interpreter. >> >> Considering the arm 64 use-case of reading PMU counters from user-space >> using rseq to prevent migration, I understand that you're lucky enough to >> already have a system call at your disposal that can perform the slow-path >> in case of single-stepping. >> >> So yes, your particular case is already covered, but unfortunately that's >> not the same situation for other use-cases that have been expressed. > > If we have users of rseq which can do without the other muck, then what's > the reason not to support it? > > The sysops thing can be sorted out on top and the use cases which need both > will have to test for both syscalls being available anyway. I'm currently making sure CONFIG_RSEQ selects both CONFIG_CPU_OPV and CONFIG_MEMBARRIER, so the user-space fast-paths don't end up with various ways of doing the fallback/single-stepping/memory barrier handling depending on whether the kernel support each of those individually. So first of all, it reduces complexity from a user-space perspective. Moreover, with a single already needed cpu_id vs cpu_id_start field comparison in the rseq fast-path, user-space knows that it can rely on having rseq, cpu_opv, and membarrier. Without this guarantee, user-space would have to detect individually whether each of those system calls is available, and test flags on the fast-path, for additional overhead. Those are my main concerns about pushing an incomplete solution at this stage. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
