[Bug 1871842] Re: AMD CPUID leaf 0x8000'0008 reported number of cores inconsistent with ACPI.MADT

[Thomas Huth]

If I got that right, there were some patches proposed for this bug ...
has this been fixed already? Or is there still anything left to do?

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

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Title:
  AMD CPUID leaf 0x8000'0008 reported number of cores  inconsistent with
  ACPI.MADT

Status in QEMU:
  Incomplete

Bug description:
  Setup:
  CPU: AMD EPYC-v2 or host's EPYC cpu
  Linux 64-bit fedora host; Kernel version 5.5.15-200.fc31
  qemu version: self build
  git-head: f3bac27cc1e303e1860cc55b9b6889ba39dee587
  config: Configured with: '../configure' 
'--target-list=x86_64-softmmu,mips64el-softmmu,mips64-softmmu,mipsel-softmmu,mips-softmmu,i386-softmmu,aarch64-softmmu,arm-softmmu'
 '--prefix=/opt/qemu-master'

  Cmdline: 
  qemu-system-x86_64 -kernel 
/home/peppelt/code/l4/internal/.build-x86_64/bin/amd64_gen/bootstrap -append "" 
-initrd "./fiasco/.build-x86_64/fiasco , ... " -serial stdio -nographic 
-monitor none -nographic -monitor none -cpu EPYC-v2 -m 4G -smp 4 

  Issue:
  We are developing an microkernel operating system called L4Re. We recently 
got an AMD EPYC server for testing and we couldn't execute SMP tests of our 
system when running Linux + qemu + VM w/ L4Re.
  In fact, the kernel did not recognize any APs at all. On AMD CPUs the kernel 
checks for the number of cores reported in CPUID leaf 0x8000_0008.ECX[NC] or 
[ApicIdSize].  [0][1]

  The physical machine reports for leaf 0x8000_0008:  EAX: 0x3030 EBX: 
0x18cf757 ECX: 0x703f EDX: 0x1000
  The lower four bits of ECX are the [NC] field and all set.

  When querying inside qemu with -enable-kvm -cpu host -smp 4 (basically as 
replacement and addition to the above cmdline) the CPUID leaf shows: EAX: 
0x3024, EBX: 0x1001000, ECX: 0x0, EDX: 0x0
  Note, ECX is zero. Indicating that this is no SMP capabale CPU.

  I'm debugging it using my local machine and the QEMU provided EPYC-v2
  CPU model and it is reproducible there as well and reports:  EAX:
  0x3028, EBX: 0x0, ECX: 0x0, EDX: 0x0

  I checked other AMD based CPU models (phenom, opteron_g3/g5) and they behave 
the same. [2] shows the CPUID 0x8000'0008 handling in the QEMU source.
  I believe that behavior here is wrong as ECX[NC] should report the number of 
cores per processor, as stated in the AMD manual [2] p.584. In my understanding 
-smp 4 should then lead to ECX[NC] = 0x3.

  The following table shows my findings with the -smp option:
  Option | Qemu guest observed ECX value
  -smp 4 | 0x0
  -smp 4,cores=4  | 0x3
  -smp 4,cores=2,thread=2 | 0x3
  -smp 4,cores=4,threads=2 | QEMU boot error: topology false.

  Now, I'm asking myself how the terminology of the AMD manual maps to QEMU's 
-smp option.
  Obviously, nr_cores and nr_threads correspond to the cores and threads 
options on the cmdline and cores * threads <= 4 (in this example), but what 
corresponds the X in -smp X to?

  Querying 0x8000'0008 on the physical processor results in different
  reports than quering QEMU's model as does it with -enable-kvm -cpu
  host.

  Furthermore, the ACPI.MADT shows 4 local APICs to be present while the
  CPU leave reports a single core processor.

  This leads me to the conclusion that CPUID 0x8000'0008.ECX reports the
  wrong number.

  
  Please let me know, if you need more information from my side.

  
  [0] 
https://github.com/kernkonzept/fiasco/blob/522ccc5f29ab120213cf02d71328e2b879cbbd19/src/kern/ia32/kernel_thread-ia32.cpp#L109
  [1] 
https://github.com/kernkonzept/fiasco/blob/522ccc5f29ab120213cf02d71328e2b879cbbd19/src/kern/ia32/cpu-ia32.cpp#L1120
  [2] 
https://github.com/qemu/qemu/blob/f2a8261110c32c4dccd84e774d8dd7a0524e00fb/target/i386/cpu.c#L5835
  [3] https://www.amd.com/system/files/TechDocs/24594.pdf

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