12:30 "[ISA] do not matter very much"

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), February 23, 2021 1:30 pm
Room: Moderated Discussions
rwessel (rwessel.delete@this.yahoo.com) on February 23, 2021 12:21 pm wrote:
>
> Is that really a problem? Just check for aliasing at startup, and as each operand
> crosses page boundaries.

It's been a problem. Look up the historical performance profiles of various rep movs ERMS implementations, and they have things like "if the source and destination address overlap in 64 bytes modulo 4096", it slows down.

I forget the exact details, but it's been an actual issue.

> > Same goes for MMIO memory. If you do memcpy() on MMIO memory, you get whatever random
> > end results. But for movsb it's actually acrhitecturally defined, and usually not
> > what you want (ie the definition is the "go slow, one byte at a time").
>
> I don't know how you get around that. If you do a byte access to MMIO memory, the hardware *has* to do
> byte accesses.

That's my point.

The way you get around it is that you define the instruction not as a "byte copy loop", but as a "memcpy". Which basically requires a new instruction with subtly different semantics.

That's what I said. Read my suggestion again.

"rep movs" is almost perfect. But it has real and present problems. And MMIO is one of them. DF is another. Physical aliasing is a third.

None of these are common, but they happen, and they cause problems.

Linus
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TopicPosted ByDate
New Lex Fridman interview with Jim KellerJohnG2021/02/20 12:25 AM
  12:30 "[ISA] do not matter very much" (NT)Moritz2021/02/20 10:45 AM
    AAARGH, what are we going to argue about then? (NT)j2021/02/20 03:43 PM
    Blasphemy! (NT):]2021/02/21 05:49 AM
    12:30 "[ISA] do not matter very much"anon22021/02/21 11:25 PM
      12:30 "[ISA] do not matter very much"Brett2021/02/22 12:59 AM
        12:30 "[ISA] do not matter very much"Etienne Lorrain2021/02/22 02:17 AM
      12:30 "[ISA] do not matter very much"Dummond D. Slow2021/02/22 09:57 AM
        12:30 "[ISA] do not matter very much"Anon2021/02/22 11:52 AM
          12:30 "[ISA] do not matter very much"juanrga2021/02/22 12:01 PM
          12:30 "[ISA] do not matter very much"Mark Roulo2021/02/22 12:54 PM
          ARM being a good idea doesn't mean it would have worked for AMDDummond D. Slow2021/02/22 02:34 PM
            ARM being a good idea doesn't mean it would have worked for AMDAnon2021/02/22 04:25 PM
              ARM being a good idea doesn't mean it would have worked for AMDDummond D. Slow2021/02/22 05:55 PM
                ARM being a good idea doesn't mean it would have worked for AMDDoug S2021/02/23 01:03 PM
                  ARM being a good idea doesn't mean it would have worked for AMDDummond D. Slow2021/02/23 01:27 PM
                    ARM being a good idea doesn't mean it would have worked for AMDBrett2021/02/23 04:57 PM
                      3rd parties licensing ARM coresAnon2021/02/25 05:01 AM
                        3rd parties licensing ARM coresAnon2021/02/25 05:48 AM
                        3rd parties licensing ARM coresdmcq2021/02/25 07:01 AM
                          3rd parties licensing ARM coresDummond D. Slow2021/02/25 10:17 AM
                            3rd parties licensing ARM coresAnon2021/02/25 11:11 AM
                              3rd parties licensing ARM coresAnon2021/02/26 03:54 AM
                              3rd parties licensing ARM coresDummond D. Slow2021/02/26 11:01 AM
            ARM being a good idea doesn't mean it would have worked for AMDLinus Torvalds2021/02/22 06:06 PM
              ARM being a good idea doesn't mean it would have worked for AMDDummond D. Slow2021/02/22 08:19 PM
              ARM being a good idea doesn't mean it would have worked for AMDanon22021/02/22 08:28 PM
              ARM being a good idea doesn't mean it would have worked for AMDdmcq2021/02/23 06:35 AM
                ARM being a good idea doesn't mean it would have worked for AMDJukka Larja2021/02/23 08:12 AM
                  ARM being a good idea doesn't mean it would have worked for AMDSimon Farnsworth2021/02/23 09:42 AM
                    ARM being a good idea doesn't mean it would have worked for AMDJukka Larja2021/02/24 07:03 AM
                ARM may have been a threat to Intelwumpus2021/02/23 09:30 AM
      12:30 "[ISA] do not matter very much"blaine2021/02/22 10:37 AM
        12:30 "[ISA] do not matter very much"anon22021/02/22 08:17 PM
          12:30 "[ISA] do not matter very much"Anon2021/02/23 04:05 AM
            12:30 "[ISA] do not matter very much"Wilco2021/02/23 04:48 AM
              12:30 "[ISA] do not matter very much"Bigos2021/02/23 04:55 AM
                12:30 "[ISA] do not matter very much"Wilco2021/02/23 05:15 AM
                  12:30 "[ISA] do not matter very much"Bigos2021/02/23 06:16 AM
                12:30 "[ISA] do not matter very much"Travis Downs2021/02/27 12:46 AM
              12:30 "[ISA] do not matter very much"Anon2021/02/23 07:26 AM
                12:30 "[ISA] do not matter very much"anon22021/02/23 05:35 PM
                  12:30 "[ISA] do not matter very much"Anon2021/02/24 08:57 AM
                12:30 "[ISA] do not matter very much"Wilco2021/02/24 05:37 AM
                  12:30 "[ISA] do not matter very much"Etienne Lorrain2021/02/24 07:24 AM
                    12:30 "[ISA] do not matter very much"Anon2021/02/24 09:11 AM
                    12:30 "[ISA] do not matter very much"rwessel2021/02/24 09:45 AM
                      12:30 "[ISA] do not matter very much"Etienne Lorrain2021/02/25 02:02 AM
                        12:30 "[ISA] do not matter very much"rwessel2021/02/25 05:51 AM
                        12:30 "[ISA] do not matter very much"Anon2021/02/25 05:53 AM
                  12:30 "[ISA] do not matter very much"Anon2021/02/24 09:07 AM
                    12:30 "[ISA] do not matter very much"Wilco2021/02/24 12:37 PM
                      runtime selection vs. heterogenous cores?Matt Sayler2021/02/24 07:10 PM
                        runtime selection vs. heterogenous cores?Wilco2021/02/26 06:22 AM
            12:30 "[ISA] do not matter very much"anon22021/02/23 05:20 AM
              12:30 "[ISA] do not matter very much"Anon2021/02/23 07:21 AM
                12:30 "[ISA] do not matter very much"none2021/02/23 08:37 AM
                  12:30 "[ISA] do not matter very much"rwessel2021/02/23 10:44 AM
                    12:30 "[ISA] do not matter very much"anon22021/02/23 05:30 PM
                      12:30 "[ISA] do not matter very much"Anon2021/02/24 09:25 AM
                        12:30 "[ISA] do not matter very much"anon.12021/02/25 07:13 AM
                  12:30 "[ISA] do not matter very much"Anon2021/02/24 09:44 AM
                12:30 "[ISA] do not matter very much"anon22021/02/23 04:51 PM
                  12:30 "[ISA] do not matter very much"Anon2021/02/24 09:31 AM
            12:30 "[ISA] do not matter very much"vvid2021/02/23 07:41 AM
              12:30 "[ISA] do not matter very much"Michael S2021/02/23 09:52 AM
                12:30 "[ISA] do not matter very much"rwessel2021/02/23 10:33 AM
                  12:30 "[ISA] do not matter very much"Linus Torvalds2021/02/23 12:44 PM
                    12:30 "[ISA] do not matter very much"rwessel2021/02/23 01:21 PM
                      12:30 "[ISA] do not matter very much"Linus Torvalds2021/02/23 01:30 PM
                        12:30 "[ISA] do not matter very much"Andrey2021/02/25 04:06 AM
                          12:30 "[ISA] do not matter very much"Anon2021/02/25 06:04 AM
                            12:30 "[ISA] do not matter very much"Andrey2021/02/25 06:54 AM
                              12:30 "[ISA] do not matter very much"Anon2021/02/25 07:33 AM
                          12:30 "[ISA] do not matter very much"Linus Torvalds2021/02/25 11:35 AM
                            12:30 "[ISA] do not matter very much"Andrey2021/02/25 02:34 PM
                              12:30 "[ISA] do not matter very much"Etienne Lorrain2021/02/26 02:18 AM
                              12:30 "[ISA] do not matter very much"dmcq2021/02/26 04:23 PM
                12:30 "[ISA] do not matter very much"Anon2021/02/24 09:45 AM
              12:30 "[ISA] do not matter very much"Gabriele Svelto2021/02/23 10:15 AM
          Context of ISA doesn't matterPaul A. Clayton2021/02/26 01:03 PM
  Is there a text version? (NT)Foo_2021/02/20 05:33 PM
    good question (NT)Michael S2021/02/21 05:31 AM
    Is there a text version?:]2021/02/21 11:34 AM
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