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

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), February 23, 2021 11:44 am
Room: Moderated Discussions
rwessel (rwessel.delete@this.yahoo.com) on February 23, 2021 9:33 am wrote:
> No one would object to rep/movsb being slow if the operands overlap.

Noe that one problem with rep/movsb is that "overlap" is hard to figure out.

They might not overlap in virtual memory, but still overlap in physical pages.

And while that doesn't matter for memcpy, it does matter for movsb, which is technically defined even for that case.

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").

So when memcpy can make decisions based on just comparing addresses and can say "screw it" to both physical aliasing and MMIO, movsb needs to actually do a TLB probe.

I think "rep movsb" is really really close to being the perfect hardware interface for "memcpy", but the above issues and the DF bit do make for it being much harder to just generate simpler optimal ucode.

So the best option might be to specify a new instruction that looks exactly like "rep movsb" but specifies that DF is ignored, and that it might be moving things in bigger chunks (so that MMIO and physical aliases get the "memcpy" semantics, not the "byte at a time" ones).

One potential way would be to say "doubled-up rep prefix means new semantics", and have a bit in CR4 enable it.

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