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Diffstat (limited to 'results/scraper/launchpad-without-comments/1871842')
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diff --git a/results/scraper/launchpad-without-comments/1871842 b/results/scraper/launchpad-without-comments/1871842 new file mode 100644 index 000000000..78267165c --- /dev/null +++ b/results/scraper/launchpad-without-comments/1871842 @@ -0,0 +1,51 @@ +AMD CPUID leaf 0x8000'0008 reported number of cores inconsistent with ACPI.MADT + +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 \ No newline at end of file |