TOPOEXT and CentOs 7 guests

[Ani Sinha]

Hi :

I am looking at a patch where we disable TOPOEXT when -cpu host or -cpu max is 
passed to qemu :

if (cpu->max_features) {
   for (w = 0; w < FEATURE_WORDS; w++) {
     /* Override only features that weren't set explicitly
      * by the user.
      */
      env->features[w] |=
      x86_cpu_get_supported_feature_word(w, cpu->migratable) &
         ~env->user_features[w] & \
         ~feature_word_info[w].no_autoenable_flags;
    }
}

https://lists.nongnu.org/archive/html/qemu-devel/2018-08/msg01641.html

We are using a setup where we pass “kvm64” as the cpu model along with other 
hypervisor CPIUD capabilities as detected by libvirt to a centOS 7.7 guest and 
the guest is unable to boot. We are using a AMD EPYC platform and we have 
traced it down to TOPOEXT flag being the offending CPUID from the host CPU 
which is causing the issue. Does it makes sense to not enable this flag by 
default on all other guest CPU models as well except for EPYC and EPTC-IBPB?

Just looking at the code very recently and thought I’d get an opinion from the 
wiser qemu community.

Thanks
Ani