Plausible, I suppose pytorch could ignore our flags and build something with vector extensions on (unless cc-wrapper filters those, I'm not sure), but what part of the logs suggested this conclusion?

Searching for "qemu avx" I see https://superuser.com/a/454814 suggesting -cpu sandyBridge,+avx,enforce

Yeah I've done all of that, and lspcu inside the vm shows

Architecture:             x86_64
  CPU op-mode(s):         32-bit, 64-bit
  Address sizes:          48 bits physical, 48 bits virtual
  Byte Order:             Little Endian
CPU(s):                   8
  On-line CPU(s) list:    0-7
Vendor ID:                AuthenticAMD
  BIOS Vendor ID:         QEMU
  Model name:             AMD Ryzen 9 3900X 12-Core Processor
    BIOS Model name:      pc-i440fx-9.1  CPU @ 2.0GHz
    BIOS CPU family:      1
    CPU family:           23
    Model:                113
    Thread(s) per core:   1
    Core(s) per socket:   8
    Socket(s):            1
    Stepping:             0
    BogoMIPS:             7599.99
    Flags:                fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge m
                          ca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall
                           nx mmxext fxsr_opt pdpe1gb rdtscp lm rep_good nopl cp
                          uid extd_apicid tsc_known_freq pni pclmulqdq ssse3 fma
                           cx16 sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_t
                          imer aes xsave avx f16c rdrand hypervisor lahf_lm cmp_
                          legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefe
                          tch osvw perfctr_core ssbd ibpb stibp vmmcall fsgsbase
                           tsc_adjust bmi1 avx2 smep bmi2 rdseed adx smap clflus
                          hopt clwb sha_ni xsaveopt xsavec xgetbv1 clzero xsavee
                          rptr wbnoinvd arat npt lbrv nrip_save tsc_scale vmcb_c
                          lean flushbyasid pausefilter pfthreshold v_vmsave_vmlo
                          ad vgif umip rdpid overflow_recov succor arch_capabili
                          ties
Virtualization features:

avx ... but what part of the logs suggested this conclusion?

It is one of these, but we do not know which one it is

          DevicePolicy = "closed";
          LockPersonality = true;
          MemoryDenyWriteExecute = true;
          NoNewPrivileges = true;
          PrivateDevices = false; # hides acceleration devices
          PrivateTmp = true;
          PrivateUsers = true;
          ProcSubset = "all"; # /proc/meminfo
          ProtectClock = true;
          ProtectControlGroups = true;
          ProtectHome = true;
          ProtectHostname = true;
          ProtectKernelLogs = true;
          ProtectKernelModules = true;
          ProtectKernelTunables = true;
          ProtectProc = "invisible";
          ProtectSystem = "strict";
          RemoveIPC = true;
          RestrictNamespaces = true;
          RestrictRealtime = true;
          RestrictSUIDSGID = true;
          RestrictAddressFamilies = [
            "AF_INET"
            "AF_INET6"
            "AF_UNIX"
          ];
          SupplementaryGroups = [ "render" ]; # for rocm to access /dev/dri/renderD* devices
          SystemCallArchitectures = "native";
          SystemCallFilter = [
            "@system-service @resources"
            "~@privileged"
          ];
          UMask = "0077";