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[Bug 1902451] Re: incorrect cpuid feature detection

From: Launchpad Bug Tracker
Subject: [Bug 1902451] Re: incorrect cpuid feature detection
Date: Fri, 09 Jul 2021 04:17:28 -0000

[Expired for QEMU because there has been no activity for 60 days.]

** Changed in: qemu
       Status: Incomplete => Expired

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  incorrect cpuid feature detection

Status in QEMU:

Bug description:

  I am currently developing a x64 kernel and I wanted to check through
  cpuid if some features are available in the guest. When I try to
  enable cpu features like vmcb_clean or constant_tsc qemu is saying
  that my host doesn't support the requested features. However cat
  /proc/cpuinfo tells a different story:

  model name:  AMD Ryzen 5 3500U
  flags         : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov 
pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb 
rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf 
pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 movbe popcnt aes xsave avx 
f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 
3dnowprefetch osvw skinit wdt tce topoext perfctr_core perfctr_nb bpext 
perfctr_llc mwaitx cpb hw_pstate sme pti ssbd sev ibpb vmmcall fsgsbase bmi1 
avx2 smep bmi2 rdseed adx smap clflushopt sha_ni xsaveopt xsavec xgetbv1 xsaves 
clzero irperf xsaveerptr arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean 
flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif 
overflow_recov succor smca

  I also checked it myself by running cpuid and check the bits as in the
  AMD Manual. Everything checks out but qemu still fails.

  QEMU version: QEMU emulator version 4.2.0

  $ qemu-system-x86_64 -cpu host,+vmcb_clean,enforce -enable-kvm -drive 
format=raw,file=target/x86_64-os/debug/bootimage-my_kernel.bin -serial stdio 
-display none
  qemu-system-x86_64: warning: host doesn't support requested feature: 
CPUID.8000000AH:EDX.vmcb-clean [bit 5]
  qemu-system-x86_64: Host doesn't support requested features


  $ qemu-system-x86_64 -cpu host,+constant_tsc,enforce -enable-kvm -drive 
format=raw,file=target/x86_64-os/debug/bootimage-my_kernel.bin -serial stdio 
-display none
  qemu-system-x86_64: Property '.constant_tsc' not found

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