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

[Babu Moger]

Thanks. I saw the update in the thread.
https://lore.kernel.org/qemu-devel/address@hidden/
Looks like you have found a way to take care of your problem.
But We need to fix the CPUID leaf 0x8000'0008 anyways.
Will send the patch to review later this week. Thanks


On 4/9/20 12:48 PM, Babu Moger wrote:
> 
> 
> On 4/9/20 9:00 AM, Igor Mammedov wrote:
>> On Thu, 09 Apr 2020 12:58:11 -0000
>> Philipp Eppelt <address@hidden> wrote:
>>
>>> Public bug reported:
>>>
>>> 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?
>> I'd say X corresponds to number of logical CPUs.
>> Depending on presence of other options these are distributed among them in 
>> magical manner
>> (see pc_smp_parse() for starters)
>>
>>> 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.
>> it matches -smp X as it should be.
>>
>>>
>>> This leads me to the conclusion that CPUID 0x8000'0008.ECX reports the
>>> wrong number.
>> CCed author of recent epyc patches who might know how AMD should work better 
>> than me.
> 
> Hmm.. Interesting.. Not sure why this did not come up during my testing.
> Probably this cpuid information is not widely used.
> 
> Yes. I am looking at it right now. I see that EPYC model is reporting
> wrong. Not sure why -cpu host is reporting wrong. I thought -cpu host gets
> the information directly from the host. Will investigate.
> 
>