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

[Babu Moger]

Philipp,
  Can you please check if this patch works for you.

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 90ffc5f..e467fee 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -5831,10 +5831,17 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t
index, uint32_t count,
         }
         *ebx = env->features[FEAT_8000_0008_EBX];
         *ecx = 0;
-        *edx = 0;
         if (cs->nr_cores * cs->nr_threads > 1) {
-            *ecx |= (cs->nr_cores * cs->nr_threads) - 1;
+            unsigned long max_apicids, bits_required;
+
+            max_apicids = (cs->nr_cores * cs->nr_threads) - 1;
+            if (max_apicids) {
+                /* Find out the number of bits to represent all the
apicids */
+                bits_required = find_last_bit(&max_apicids,
BITS_PER_BYTE) + 1;
+                *ecx |= bits_required << 12 | max_apicids;
+            }
         }
+        *edx = 0;
         break;
     case 0x8000000A:
         if (env->features[FEAT_8000_0001_ECX] & CPUID_EXT3_SVM) {



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.
> 
>>
>> Please let me know, if you need more information from my side.
>>
>>
>> [0] 
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fkernkonzept%2Ffiasco%2Fblob%2F522ccc5f29ab120213cf02d71328e2b879cbbd19%2Fsrc%2Fkern%2Fia32%2Fkernel_thread-ia32.cpp%23L109&amp;data=02%7C01%7Cbabu.moger%40amd.com%7C57569f7959744399655b08d7dc8e6e24%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637220379083511672&amp;sdata=hcFJzLAVQoIh5IN9CP%2F9cUQNOZoBnpRA6FliJur1wzQ%3D&amp;reserved=0
>> [1] 
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fkernkonzept%2Ffiasco%2Fblob%2F522ccc5f29ab120213cf02d71328e2b879cbbd19%2Fsrc%2Fkern%2Fia32%2Fcpu-ia32.cpp%23L1120&amp;data=02%7C01%7Cbabu.moger%40amd.com%7C57569f7959744399655b08d7dc8e6e24%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637220379083511672&amp;sdata=ANJIbYKbwfq2bDelH%2FRLKnDPIUZc1BwxHspmgxLU7gs%3D&amp;reserved=0
>> [2] 
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fqemu%2Fqemu%2Fblob%2Ff2a8261110c32c4dccd84e774d8dd7a0524e00fb%2Ftarget%2Fi386%2Fcpu.c%23L5835&amp;data=02%7C01%7Cbabu.moger%40amd.com%7C57569f7959744399655b08d7dc8e6e24%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637220379083511672&amp;sdata=oj3mv9e5YOzUsfUjXK44gC8LybyWgMKo8JBIrRR%2BmDA%3D&amp;reserved=0
>> [3] 
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.amd.com%2Fsystem%2Ffiles%2FTechDocs%2F24594.pdf&amp;data=02%7C01%7Cbabu.moger%40amd.com%7C57569f7959744399655b08d7dc8e6e24%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637220379083511672&amp;sdata=7Yr3J9ihlqSqXCXKN5JJNTByO3NGI%2BGMz2EqBF2Y4hw%3D&amp;reserved=0
>>
>> ** Affects: qemu
>>      Importance: Undecided
>>          Status: New
>>
>