On March 3, 2016 10:35:50 AM PST, "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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 <decui@xxxxxxxxxxxxx> 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/20160112150032-mutt-send-email-mst@xxxxxxxxxx
>>
>> And some useful bits here:
>>
>> lkml.kernel.org/r/56957D54.5000602@xxxxxxxxx
>>
>> latest version here:
>>
>> lkml.kernel.org/r/1453921746-16178-1-git-send-email-mst@xxxxxxxxxxx
>
>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.
--
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