On Fri, Oct 06, 2017 at 11:08:25PM +0200, Peter Zijlstra wrote: > On Fri, Oct 06, 2017 at 08:57:56PM +0000, Mathieu Desnoyers wrote: > > Hi Hans, > > > > I'm currently making sure the > > MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED command makes its way into > > the 4.14 kernel before the end of the release candidates. Once that > > is done, I plan to post a patch adding a new MEMBARRIER_FLAG_SYNC_CORE > > flag for the 4.15 merge window. > > > > I have done a bit of research on the various architecture requirements > > for core serialization. Here are my findings so far about > > instructions providing core serialization on the main architectures > > supported by Linux. > > > > There are two places where we need it: in the interrupt handler for > > the membarrier IPI, and between scheduler execution (which can change > > the current "mm") and return to user-space. > > > > Please let me know if I missed anything. > > > > x86: iret, cpuid, wbinvd -> iret currently provides core serialization > > when going back to userspace and at the end of the IPI. There are > > plans to implement a return path without iret in the future, in which > > case I would need to issue an explicit "cpuid" instruction > > (sync_core()) in switch_mm() if the process is registered with > > MEMBARRIER_FLAG_SYNC_CORE. > > I would much prefer setting a TIF flag that forces the IRET path instead > of doing additional work in switch_mm(). > > > arm32: returning to user-space provides core serialization. Same at > > the end of membarrier IPI (to be confirmed). aarch64: ERET > > instruction used when returning to user-space provides core sync. Same > > at the end of membarrier IPI (to be confirmed). > > I thought Will already confirmed ERET did what we need, no? Yes. Will
