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Re: [Bug 1871842] [v2 PATCH] target/i386: Fix the CPUID leaf CPUID_Fn800
From: |
Philipp Eppelt |
Subject: |
Re: [Bug 1871842] [v2 PATCH] target/i386: Fix the CPUID leaf CPUID_Fn80000008 |
Date: |
Tue, 21 Apr 2020 11:45:28 -0000 |
Hi,
thanks for the patch, I tested it in my setup and I'm seeing numbers
that make sense.
However, I can create a setup which places the value 02h* into the
ApicIdSize field, which is reserved. However, I deem this a
configuration issue as well.
* -cpu EPYC-v2 -smp 4,cores=4 --> 0x8000_0008[ECX] = 0x2003
Cheers,
Philipp
On 4/17/20 11:55 PM, Babu Moger wrote:
> CPUID leaf CPUID_Fn80000008_ECX provides information about the
> number of threads supported by the processor. It was found that
> the field ApicIdSize(bits 15-12) was not set correctly.
>
> ApicIdSize is defined as the number of bits required to represent
> all the ApicId values within a package.
>
> Valid Values: Value Description
> 3h-0h Reserved.
> 4h up to 16 threads.
> 5h up to 32 threads.
> 6h up to 64 threads.
> 7h up to 128 threads.
> Fh-8h Reserved.
>
> Fix the bit appropriately.
>
> This came up during following thread.
> https://lore.kernel.org/qemu-devel/address@hidden/#t
>
> Refer the Processor Programming Reference (PPR) for AMD Family 17h
> Model 01h, Revision B1 Processors. The documentation is available
> from the bugzilla Link below.
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
>
> Reported-by: Philipp Eppelt <address@hidden>
> Signed-off-by: Babu Moger <address@hidden>
> ---
> v2:
> Used env->pkg_offset for bits 15:12 which is already available.
>
> target/i386/cpu.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 90ffc5f..5e5a605 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -5830,11 +5830,20 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index,
> uint32_t count,
> *eax = cpu->phys_bits;
> }
> *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;
> + /*
> + * Bits 15:12 is "The number of bits in the initial
> + * Core::X86::Apic::ApicId[ApicId] value that indicate
> + * thread ID within a package". This is already stored at
> + * CPUX86State::pkg_offset.
> + * Bits 7:0 is "The number of threads in the package is NC+1"
> + */
> + *ecx = (env->pkg_offset << 12) |
> + ((cs->nr_cores * cs->nr_threads) - 1);
> + } else {
> + *ecx = 0;
> }
> + *edx = 0;
> break;
> case 0x8000000A:
> if (env->features[FEAT_8000_0001_ECX] & CPUID_EXT3_SVM) {
>
--
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https://bugs.launchpad.net/bugs/1871842
Title:
AMD CPUID leaf 0x8000'0008 reported number of cores inconsistent with
ACPI.MADT
Status in QEMU:
New
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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