[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v11 1/5] xen: move xen_setup_runstate_info and get_runstate_snapshot to drivers/xen/time.c
On Thu, 5 Nov 2015, Mark Rutland wrote: > Hi, > > > +static u64 get64(const u64 *p) > > +{ > > + u64 ret; > > + > > + if (BITS_PER_LONG < 64) { > > + u32 *p32 = (u32 *)p; > > + u32 h, l; > > + > > + /* > > + * Read high then low, and then make sure high is > > + * still the same; this will only loop if low wraps > > + * and carries into high. > > + * XXX some clean way to make this endian-proof? > > + */ > > + do { > > + h = p32[1]; > > + barrier(); > > + l = p32[0]; > > + barrier(); > > + } while (p32[1] != h); > > I realise this is simply a move of existing code, but it may be better > to instead have: > > do { > h = READ_ONCE(p32[1]); > l = READ_ONCE(p32[0]); > } while (READ_ONCE(p32[1] != h); > > Which ensures that each load is a single access (though it almost > certainly would be anyway), and prevents the compiler from having to > reload any other memory locations (which the current barrier() usage > forces). I am happy to make these changes, however for code clarity and review simplicity I'll keep them on a separate patch (I like code movement to remain code movement). I can squash the two patches together when committing, if necessary. > > + > > + ret = (((u64)h) << 32) | l; > > + } else > > + ret = *p; > > Likewise, this would be better as READ_ONCE(*p), to force a single > access. > > > + > > + return ret; > > +} > > > + do { > > + state_time = get64(&state->state_entry_time); > > + barrier(); > > + *res = *state; > > + barrier(); > > You can also have: > > *res = READ_ONCE(*state); > > That will which will handle the barriers implicitly. > > Thanks, > Mark. > > > + } while (get64(&state->state_entry_time) != state_time); > > +} > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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