On Tuesday 10 June 2008 17:57, Jeremy Fitzhardinge wrote: > Nick Piggin wrote: > > On Tuesday 10 June 2008 17:35, Isaku Yamahata wrote: > >> This patch is ported one from 534:77db69c38249 of linux-2.6.18-xen.hg. > >> Use wmb instead of rmb to enforce ordering between > >> evtchn_upcall_pending and evtchn_pending_sel stores > >> in xen_evtchn_do_upcall(). > > > > There are a whole load of places in the kernel that should be using > > smp_ variants of memory barriers. This seemed to me like one of them, > > but I could be wrong. > > No, it needs to be an unconditional barrier. This is synchronizing with > the hypervisor - even if the kernel is compiled UP, the SMP hypervisor > may be testing/setting the events pending bits from another (physical) cpu. OK. What you *really* want is smp_*mb_even_if_compiled_for_UP() ;) That is, a small set of primitives that are compiled with CONFIG_SMP (and given some xxx_ prefix to distinguish). IO barriers are probably the best thing you can use for the moment. > > Also, if you do that can you get rid of the ifdef? If it really *really* > > mattered, we could introduce smp_mb before/after xchg... but if you > > use smp_wmb anyway then it definitely does not matter because that is a > > noop on x86. > > Yes, I'd like to lose the #ifdef. Unfortunately I think putting a > "lock; addl $0,0(%%esp)" style barrier had a measurable negative > performance impact, but I may be thinking of something else. I don't > know how expensive sfence is. > > The alternative is to make ia64's xchg a barrier (or to add a barrier > variant of it). It seems like a wart to have a cross-architecture > function like xchg(), but then have different architectures differ in > important details like barrier-ness. Well, no you have to be careful. Because even if we did ask for ia64's xchg to be a full barrier, you wouldn't get the right behaviour on UP because it would be free to optimise that away -- those kinds of barriers referred to in all those primitives are defined for cacheable memory and CPU-to-CPU only... I guess under the circumstances, leaving the ifdef there is probably reasonable. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
