On Thu, Mar 03, 2016 at 04:34:53PM +0100, Peter Zijlstra wrote: > On Thu, Mar 03, 2016 at 04:27:39PM +0100, Ingo Molnar wrote: > > > > * Dexuan Cui <[email protected]> wrote: > > > > > Hi, > > > My understanding about arch/x86/include/asm/barrier.h is: obviously Linux > > > more likes {L,S,M}FENCE -- Locked ADD is only used in x86_32 platforms > > > that > > > don't support XMM2. > > > > > > However, it looks people say Locked Add is much faster than the FENCE > > > instructions, even on modern Intel CPUs like Haswell, e.g., please see > > > the three sources: > > > > > > " 11.5.1 Locked Instructions as Memory Barriers > > > Optimization > > > Use locked instructions to implement Store/Store and Store/Load barriers. > > > " > > > http://support.amd.com/TechDocs/47414_15h_sw_opt_guide.pdf > > > > > > "lock addl %(rsp), 0 is a better solution for StoreLoad barrier ": > > > http://shipilev.net/blog/2014/on-the-fence-with-dependencies/ > > > > > > "...locked instruction are more efficient barriers...": > > > http://www.pvk.ca/Blog/2014/10/19/performance-optimisation-~-writing-an-essay/ > > > > > > I also found that FreeBSD prefers Locked Add. > > > > > > So, I'm curious why Linux prefers MFENCE. > > > I guess I may be missing something. > > > > > > I tried to google the question, but didn't find an answer. > > > > It's being worked on, see this thread on lkml from a few weeks ago: > > > > C Jan 13 Michael S. Tsir | [PATCH v3 0/4] x86: faster > > mb()+documentation tweaks > > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 1/4] x86: add cc clobber for > > addl > > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 2/4] x86: drop a comment left > > over from X86_OOSTORE > > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 3/4] x86: tweak the comment > > about use of wmb for IO > > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 4/4] x86: drop mfence in > > favor of lock+addl > > > > The 4th patch changes MFENCE to a LOCK ADDL locked instruction. > > Lots of additional chatter here: > > lkml.kernel.org/r/[email protected] > > And some useful bits here: > > lkml.kernel.org/r/[email protected] > > latest version here: > > lkml.kernel.org/r/[email protected]
It's ready as far as I am concerned. Basically we are just waiting for ack from hpa. -- MST
