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AMD Bulldozer/Jaguar CPUs Will No Longer Advertise RdRand Support Under Linux

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  • AMD Bulldozer/Jaguar CPUs Will No Longer Advertise RdRand Support Under Linux

    Phoronix: AMD Bulldozer/Jaguar CPUs Will No Longer Advertise RdRand Support Under Linux

    Not directly related to the recent AMD Zen 2 BIOS update needed to fix an RdRand problem (though somewhat related in that the original systemd bug report for faulty AMD RdRand stems from these earlier CPUs), but AMD has now decided to no longer advertise RdRand support for Family 15h (Bulldozer) and Family 16h (Jaguar) processors under Linux...

    http://www.phoronix.com/scan.php?pag...isable-15h-16h

  • #2
    Typo:

    Originally posted by phoronix View Post
    Tom Lendacky of AMD reesorted to clearing the RDRAND CPU ID bit for 15h/16h processors
    What about Windows? Does the same issue occur there, or did they work around it?

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    • #3
      AFAIK, the only Bulldozer-derived CPUs, that have RdRand are excavator-based APUs. So not a big deal.

      Comment


      • #4
        Originally posted by dev547 View Post
        AFAIK, the only Bulldozer-derived CPUs, that have RdRand are excavator-based APUs. So not a big deal.
        May actually be a big deal because APUs are often designed for laptops, which tend to suspend/hibernate a lot.

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        • #5
          Originally posted by tildearrow View Post

          May actually be a big deal because APUs are often designed for laptops, which tend to suspend/hibernate a lot.
          Yep, this problem happened on my brother's laptop. and it was unusable. Could not start/restart any systemd services. GNOME Terminal launches a systemd service for some reason, and it silently fails. Lots of subtle, silent bugs with no error indication, until the whole system locks up.

          It's very bad for users.

          And yeah I'm interested in knowing whether it works fine on Windows. IMO this was a defective product shipped to users, and AMD and system manufacturers owe their customers a fix for this bug.

          Once again, shoddy Linux support from AMD who are more interested in disabling these features or covering them up, rather than standing by users. Once again, lower confidence in AMD's Linux support.

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          • #6
            IMHO it is unacceptable that this took so long (years!) and is not even properly fixed in BIOS / microcode. Just switching an previously advertised features of is not a fix.

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            • #7
              So would Coreboot be a solution, if it was ready to replace BIOS for these particular CPUs /laptops? Maybe an opportunity to shine...

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              • #8
                Neither Kaveri nor Jaguar supported rdrand anyway .

                Not sure maybe Excavator and Puma had it in cpuinfo

                edit: Yeah, just checked on Athlon 5350 machine, rdrand is not there... it is last of these not plagued by PSP Plague started after that with Puma
                Last edited by dungeon; 08-15-2019, 07:29 AM.

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                • #9
                  Originally posted by sandy8925 View Post
                  Once again, shoddy Linux support from AMD who are more interested in disabling these features or covering them up, rather than standing by users. Once again, lower confidence in AMD's Linux support.
                  Did you get that AMD is just one small part in the solution chain?

                  They're releasing a CPU, microcode, some other smal pieces. They sell that stuff to OEMs, which are sourcing accompanying hardware and software (bios,...) from other vendors. Then they're blending and glueing all that stuff together. After all, PCs are a modular system. All these glue and blend steps will yield additional bugs, all the time. If you want to get rid of a bug, a lot of groups have to work together. If one group doesn't care, you're out of luck.

                  Part of this equation:
                  If AMD did do all things right and the OEM does something wrong, that's simply not AMDs fault. You should contact your OEM and blame them. If the OEM states your usecase (linux, dish washing, whatever) isn't supported, you are out of luck.

                  And:
                  If vendors decide to do their own stuff, like not validating linux due to cost or relevance, or to give customers "unique USPs" like advertising PCIE 4.0 (recent example, AMD did announce before no vendor should do that for old chipset based motherboards...)... that's your OEMs fault.

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                  • #10
                    Originally posted by tildearrow View Post
                    Typo:

                    What about Windows? Does the same issue occur there, or did they work around it?
                    The real question is: does windows use it at all?
                    On Linux, unless explicitly used, RdRand just should go on a pile of entropy served by the kernel, because we really don't trust these kind of "trust us it is really random" generators. So random values in the linux kernel are never determined by RdRand. Ever. So even if it just returns -1, it does not matter.
                    It does matter however if applications use it directly. So the patch of AMD is correct, it actually says: do not rely on this hardware, use the linux kernel instead.

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