memory in ubuntu_kvm_unit_tests fails (clflushopt / clwb ABSENT)

Passed with Oracle Bionic 4.15

Oops, spoke too soon.

This has passed with Oracle Bionic 4.15 on VM.DenseIO1.8, but failed on VM.DenseIO2.16

Passed on VM.DenseIO1.8 and VM.Standard1.4 for Oracle B-4.15

see the same sort of failure on E-linux-kvm

12/09 11:58:32 DEBUG| utils:0116| Running 'kvm-ok'
12/09 11:58:32 DEBUG| utils:0153| [stdout] INFO: /dev/kvm exists
12/09 11:58:32 DEBUG| utils:0153| [stdout] KVM acceleration can be used
12/09 11:58:32 DEBUG| utils:0116| Running '/home/ubuntu/autotest/client/tmp/ubuntu_kvm_unit_tests/src/kvm-unit-tests/tests/memory'
12/09 11:58:32 DEBUG| utils:0153| [stdout] BUILD_HEAD=e2c275c4
12/09 11:58:32 DEBUG| utils:0153| [stdout] timeout -k 1s --foreground 90s /usr/bin/qemu-system-x86_64 -nodefaults -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device pci-testdev -machine accel=kvm -kernel /tmp/tmp.YNkf1n3ucP -smp 1 -cpu host # -initrd /tmp/tmp.6LT1XGulTr
12/09 11:58:33 DEBUG| utils:0153| [stdout] enabling apic
12/09 11:58:33 DEBUG| utils:0153| [stdout] PASS: clflush (present)
12/09 11:58:33 DEBUG| utils:0153| [stdout] PASS: sfence (present)
12/09 11:58:33 DEBUG| utils:0153| [stdout] PASS: lfence (present)
12/09 11:58:33 DEBUG| utils:0153| [stdout] PASS: mfence (present)
12/09 11:58:33 DEBUG| utils:0153| [stdout] FAIL: clflushopt (ABSENT)
12/09 11:58:33 DEBUG| utils:0153| [stdout] FAIL: clwb (ABSENT)
12/09 11:58:33 DEBUG| utils:0153| [stdout] PASS: invalid clwb
12/09 11:58:33 DEBUG| utils:0153| [stdout] PASS: pcommit (ABSENT)
12/09 11:58:33 DEBUG| utils:0153| [stdout] SUMMARY: 8 tests, 2 unexpected failures
12/09 11:58:33 DEBUG| utils:0153| [stdout] FAIL memory (8 tests, 2 unexpected failures)
12/09 11:58:33 ERROR| test:0414| Exception escaping from test:

With 4.15.0-1037.41 Oracle
Failed with VM.Standard2.16
Passed with VM.Standard2.1

failed on Bionic/azure-4.15 : 4.15.0-1097.107 : amd64

Failed on Bionic/oracle : 5.4.0-1026.26~18.04.1 : amd64

Seen with linux-oracle 4.15.0-1065.73~16.04.1.

Found on 5.11.0-1009.9+21.10.1-kvm

 timeout -k 1s --foreground 90s /usr/bin/qemu-system-x86_64 --no-reboot -nodefaults -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device pci-testdev -machine accel=kvm -kernel /tmp/tmp.RiE0Ijtuov -smp 1 -cpu max # -initrd /tmp/tmp.G3a4COl6Lh
 enabling apic
 PASS: clflush (present)
 PASS: sfence (present)
 PASS: lfence (present)
 PASS: mfence (present)
 FAIL: clflushopt (ABSENT)
 FAIL: clwb (ABSENT)
 PASS: invalid clwb
 PASS: pcommit (ABSENT)
 SUMMARY: 8 tests, 2 unexpected failures
 FAIL memory (8 tests, 2 unexpected failures)

Observed in H/KVM, cycle sru-20210531.

Observed in G/KVM, cycle sru-20210531.

Observed in F/KVM, cycle sru-20210531.

Observed in B/KVM, cycle sru-20210531.

Observed in T-4.15/Azure, cycle sru-20210531.

Found on H-kvm 5.11.0-1015.16

Status changed to 'Confirmed' because the bug affects multiple users.

When comparing the Focal 5.4 generic AMD64 kernel test output and Focal 5.4 KVM AMD64 kernel test output, you can see the generic kernel will pass with:

 PASS: clflush (present)
 PASS: sfence (present)
 PASS: lfence (present)
 PASS: mfence (present)
 PASS: clflushopt (ABSENT)
 PASS: clwb (ABSENT)
 PASS: invalid clwb
 PASS: pcommit (ABSENT)
 SUMMARY: 8 tests
 PASS memory (8 tests)

Instead the KVM kernel will fail with:
 FAIL: clflushopt (ABSENT)
 FAIL: clwb (ABSENT)

I wonder what's the difference here.