On Tue, Jan 05, 2021 at 08:20:51AM -0800, Andy Lutomirski wrote: > > Interestingly, the architecture recently added a control bit to remove > > this synchronisation from exception return, so if we set that then we'd > > have a problem with SYNC_CORE and adding an ISB would be necessary (and > > we could probable then make kernel->kernel returns cheaper, but I > > suspect we're relying on this implicit synchronisation in other places > > too). > > > > Is ISB just a context synchronization event or does it do more?
IIRC it just the instruction sync (like power ISYNC). > On x86, it’s very hard to tell that MFENCE does any more than LOCK, > but it’s much slower. And we have LFENCE, which, as documented, > doesn’t appear to have any semantics at all. (Or at least it didn’t > before Spectre.) AFAIU MFENCE is a completion barrier, while LOCK prefix is not. A bit like ARM's DSB vs DMB. It is for this reason that mb() is still MFENCE, while our smp_mb() is a LOCK prefixed NO-OP. And yes, LFENCE used to be poorly defined and it was sometimes understood to be a completion barrier relative to prior LOADs, while it is now a completion barrier for any prior instruction, and really should be renamed to IFENCE.
