[SOLVED] Problem with CPU flags

[Leah]

We have a small problem here with the cpu type option for our proxmox VMs. We run PVE 6.18 currently. The Problem is, that if we select "host" as CPU type the flags arn't the same but they should. Is there an easy explenation for that?

From the Docs:
> If you want an exact match, you can set the CPU type to host in which case the VM will have exactly the same CPU flags as your host system.

Host:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 invpcid_single pti intel_ppin ssbd ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm rdt_a rdseed adx smap intel_pt xsaveopt cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts flush_l1d

VM with CPU type host:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon rep_good nopl xtopology cpuid tsc_known_freq pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch cpuid_fault invpcid_single pti ssbd ibrs ibpb stibp fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm rdseed adx smap xsaveopt arat umip arch_capabilities

 

[fabian]

which ones are you missing? https://git.kernel.org/pub/scm/linu...x.git/tree/arch/x86/include/asm/cpufeatures.h gives an explanation which feature/flag does what. some of them simply are not transferrable to a VM..

 

[fabian]

(and some of them are not actually CPU features, but bugs or derived information that the kernel treats as a feature..)

 

[Leah]

Ahh ok...than the docs where a little misleading in this case, "exact the same" sounds like it is more comparable if you look at /proc/cpuinfo ... I guess

 

[fabian]

yes, that phrasing is a bit misleading. /proc/cpuinfo is generated by the (host/VM) kernel, so even just running different distros/versions will affect the output, and stuff like microcode, VMX nesting, things that only make sense on physical hardware like thermal/debug/.. readouts complicate matters even more.

for those flags that matter in practice, 'host' should give you a 1:1 mapping with the exception of virtualization extensions which need to be explicitly enabled to be available.