Sequential consistency in hardware

By: Travis Downs (travis.downs.delete@this.gmail.com), August 5, 2020 6:39 pm
Room: Moderated Discussions
Jeff S. (fakity.delete@this.fake.com) on August 5, 2020 3:52 pm wrote:
> Travis Downs (travis.downs.delete@this.gmail.com) on August 5, 2020 1:04 pm wrote:
> > Well my thought was that in the existing TSO model, a store forwarding is always allowed,
> > so using a stored value as the source for the load isn't subject to any verification later.
> > Of course, perhaps such loads are inserted anyway in the MOB (or whatever other structure)
> > and subject to invalidation-based nuking, in which case it isn't a problem.
> > For something like memory renaming, it does mean that the load still needs to be tracked in the
> > memory ordering structures, which seems unfortunate, because I think under x86-TSO this would not
> > be required: the forwarding is always valid in the absence of any intervening memory barriers?
>
> I am in agreement that remotely-induced squashes should not fundamentally be an extra burden for
> TSO to monitor in store-to-load forwarding, but I do think that the even forwarded require protracted
> susceptibility to squashes in the fully general case. At least until every older store has arrived
> in the MOB, to protect against intervening aliasing stores that reach the MOB later.

I am not sure why are on the same page. In this paragraph you are talking about how it could work under x86-TSO, right?

Why do you emphasize remotely in "remotely induced squashes"? What is the non-remote case?

Where the forwarding was incorrectly predicted because a store that falls between the load and the (predicted to forward) store ends up aliasing? Obviously this has to be detected, but I sort of considered it a function of the "store buffer" rather than the MOB (although how separate these structures are, physically, is open to debate).

So when you say the "fully general case" you are talking about the possibility of (core local) forwarding speculation failure?

It is not obvious to me that the same mechanism would be used to detect both remote-induced ordering violations and speculation failures, although I do note that the same PMU event (MACHINE_CLEARS.MEMORY_ORDERING) is used to count both cases.

What do you mean for a store to "arrive in the MOB"? That its address becomes known?
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TopicPosted ByDate
Sequential consistency in hardwarenever_released2020/08/03 07:44 AM
  Sequential consistency in hardwareLinus Torvalds2020/08/03 09:19 AM
    Sequential consistency in hardwareJon Masters2020/08/03 04:22 PM
      Sequential consistency in hardwareGeert Bosch2020/08/03 07:48 PM
        Sequential consistency in hardwareTravis Downs2020/08/03 08:08 PM
          Sequential consistency in hardwareLinus Torvalds2020/08/03 10:20 PM
            Sequential consistency in hardwareLinus Torvalds2020/08/04 11:56 AM
              Sequential consistency in hardwarenever_released2020/08/04 02:03 PM
            Sequential consistency in hardwareVeedrac2020/08/05 11:54 AM
              Sequential consistency in hardwareDoug S2020/08/05 02:36 PM
                Sequential consistency in hardwareanon22020/08/05 03:06 PM
          Sequential consistency in hardwareAnon2020/08/04 07:02 AM
        Sequential consistency in hardwaredmcq2020/08/04 09:27 AM
          Sequential consistency in hardwareKonrad Schwarz2020/08/05 05:03 AM
  Sequential consistency in hardwareTravis Downs2020/08/03 06:58 PM
    Sequential consistency in hardwaregpd2020/08/04 02:19 AM
    Sequential consistency in hardwareJeff S.2020/08/04 10:11 PM
      Sequential consistency in hardwareTravis Downs2020/08/05 12:04 PM
        Sequential consistency in hardwareJeff S.2020/08/05 02:52 PM
          typoJeff S.2020/08/05 02:55 PM
          Sequential consistency in hardwareTravis Downs2020/08/05 06:39 PM
            Sequential consistency in hardwareJeff S.2020/08/05 07:43 PM
  Binary translationDavid Kanter2020/08/03 08:19 PM
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