On March 3, 2016 10:35:50 AM PST, "Michael S. Tsirkin" <[email protected]> wrote: >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.
And I'm still discussing this with the hardware people. It seems we can do this for *most* things, but not all; the question is where exactly we need to do something different. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.
