Comment 28 for bug 1828495

Expected results is:
HOST and Guest should enable IA32_ARCH_CAPABILITIES MSR.
MDS_NO is bit 5 of ARCH_CAPABILITIES. Expose this bit to guest.

##cpuid -r
0x00000007 0x00: eax=0x00000000 ebx=0xd19f4fb9 ecx=0x00000818 edx=0x84000000
edx's 29 bit should be 1.

#rdmsr 0x10a -f 5:5
return value should be 1

Actual result is
host works well as expected.
guest not.

Below are details.

Host
# qemu-system-x86_64 -accel kvm -drive if=virtio,id=hd,file=ubuntu-18.04.2-server-amd64.qcow,format=qcow2 -m 4096 -smp 4 -cpu Cascadelake-Server,+arch-capabilities -serial stdio -redir tcp:2223::22

root@PLY02:~# cpuid -r |more
CPU 0:
   0x00000000 0x00: eax=0x00000016 ebx=0x756e6547 ecx=0x6c65746e edx=0x49656e69
……
   0x00000007 0x00: eax=0x00000000 ebx=0xd39ffffb ecx=0x00000818 edx=0xbc000400

edx is 0xbc000400, 29bit is 1

root@PLY02:~# rdmsr 0x10AH
2b

## 5 bit is 1

Guest

root@test:~# cpuid -r |more
CPU 0:
   0x00000000 0x00: eax=0x0000000d ebx=0x756e6547 ecx=0x6c65746e edx=0x49656e69
   ……
   0x00000007 0x00: eax=0x00000000 ebx=0xd19f4fb9 ecx=0x00000818 edx=0x84000000

## edx's 29 bit is 0

root@PLY02:~# rdmsr 0x10AH
0
## 5 bit is 0

Reason:
If you want to see the features that enumerated by MSR_IA32_ARCH_CAPABILITIES in guest with Cascadelake-Server cpu model, just using “-cpu Cascadelake-Server,+arch-capabilities” is not enough.
“-cpu Cascadelake-Server,+arch-capabilities” only let guest see MSR_IA32_ARCH_CAPABILITIES, but it doesn’t contain any feature enumerated by this msr, so the result of rdmsr 0x10a is 0.

If you want to see feature MDS_NO (bit 5) in guest, you should use “-cpu Cascadelake-Server,+arch-capabilities,+mds_no”.

Further, we get 0x2b (bit 0,1,3,5) when rdmsr 0x10a in host, which means host supports “rdctl-no”, “ibrs-all”, “skip-l1dfl-vmentry”, "mds-no".
If we want guest has the same ability as host, not only should we add arch_capabilities explicitly, but also add the features list above explicitly. Otherwise we cannot see these features in guest.

In a word, it’s all due to current Cascadelake-Server cpu model. It lacks all above.
After new version of Cascadelake-Server added in qemu, we can get rid of all these manually adding features annoyance.

Eduardo has sent out the qemu patch for versioned cpu model and patch 09 of which contains new version of Cascadelake Server cpu model. It depends on when they are merged.
https://<email address hidden>/msg627282.html