Re: [PATCH v4 03/21] softmmu: Fix CPUSTATE.nr_cores' calculation

[Zhao Liu]

Hi Philippe,

On Thu, Sep 14, 2023 at 09:31:52AM +0200, Philippe Mathieu-Daudé wrote:
> Date: Thu, 14 Sep 2023 09:31:52 +0200
> From: Philippe Mathieu-Daudé <philmd@linaro.org>
> Subject: Re: [PATCH v4 03/21] softmmu: Fix CPUSTATE.nr_cores' calculation
> 
> Hi,
> 
> On 14/9/23 09:21, Zhao Liu wrote:
> > From: Zhuocheng Ding <zhuocheng.ding@intel.com>
> > 
> >  From CPUState.nr_cores' comment, it represents "number of cores within
> > this CPU package".
> > 
> > After 003f230e37d7 ("machine: Tweak the order of topology members in
> > struct CpuTopology"), the meaning of smp.cores changed to "the number of
> > cores in one die", but this commit missed to change CPUState.nr_cores'
> > calculation, so that CPUState.nr_cores became wrong and now it
> > misses to consider numbers of clusters and dies.
> > 
> > At present, only i386 is using CPUState.nr_cores.
> > 
> > But as for i386, which supports die level, the uses of CPUState.nr_cores
> > are very confusing:
> > 
> > Early uses are based on the meaning of "cores per package" (before die
> > is introduced into i386), and later uses are based on "cores per die"
> > (after die's introduction).
> > 
> > This difference is due to that commit a94e1428991f ("target/i386: Add
> > CPUID.1F generation support for multi-dies PCMachine") misunderstood
> > that CPUState.nr_cores means "cores per die" when calculated
> > CPUID.1FH.01H:EBX. After that, the changes in i386 all followed this
> > wrong understanding.
> > 
> > With the influence of 003f230e37d7 and a94e1428991f, for i386 currently
> > the result of CPUState.nr_cores is "cores per die", thus the original
> > uses of CPUState.cores based on the meaning of "cores per package" are
> > wrong when multiple dies exist:
> > 1. In cpu_x86_cpuid() of target/i386/cpu.c, CPUID.01H:EBX[bits 23:16] is
> >     incorrect because it expects "cpus per package" but now the
> >     result is "cpus per die".
> > 2. In cpu_x86_cpuid() of target/i386/cpu.c, for all leaves of CPUID.04H:
> >     EAX[bits 31:26] is incorrect because they expect "cpus per package"
> >     but now the result is "cpus per die". The error not only impacts the
> >     EAX calculation in cache_info_passthrough case, but also impacts other
> >     cases of setting cache topology for Intel CPU according to cpu
> >     topology (specifically, the incoming parameter "num_cores" expects
> >     "cores per package" in encode_cache_cpuid4()).
> > 3. In cpu_x86_cpuid() of target/i386/cpu.c, CPUID.0BH.01H:EBX[bits
> >     15:00] is incorrect because the EBX of 0BH.01H (core level) expects
> >     "cpus per package", which may be different with 1FH.01H (The reason
> >     is 1FH can support more levels. For QEMU, 1FH also supports die,
> >     1FH.01H:EBX[bits 15:00] expects "cpus per die").
> > 4. In cpu_x86_cpuid() of target/i386/cpu.c, when CPUID.80000001H is
> >     calculated, here "cpus per package" is expected to be checked, but in
> >     fact, now it checks "cpus per die". Though "cpus per die" also works
> >     for this code logic, this isn't consistent with AMD's APM.
> > 5. In cpu_x86_cpuid() of target/i386/cpu.c, CPUID.80000008H:ECX expects
> >     "cpus per package" but it obtains "cpus per die".
> > 6. In simulate_rdmsr() of target/i386/hvf/x86_emu.c, in
> >     kvm_rdmsr_core_thread_count() of target/i386/kvm/kvm.c, and in
> >     helper_rdmsr() of target/i386/tcg/sysemu/misc_helper.c,
> >     MSR_CORE_THREAD_COUNT expects "cpus per package" and "cores per
> >     package", but in these functions, it obtains "cpus per die" and
> >     "cores per die".
> > 
> > On the other hand, these uses are correct now (they are added in/after
> > a94e1428991f):
> > 1. In cpu_x86_cpuid() of target/i386/cpu.c, topo_info.cores_per_die
> >     meets the actual meaning of CPUState.nr_cores ("cores per die").
> > 2. In cpu_x86_cpuid() of target/i386/cpu.c, vcpus_per_socket (in CPUID.
> >     04H's calculation) considers number of dies, so it's correct.
> > 3. In cpu_x86_cpuid() of target/i386/cpu.c, CPUID.1FH.01H:EBX[bits
> >     15:00] needs "cpus per die" and it gets the correct result, and
> >     CPUID.1FH.02H:EBX[bits 15:00] gets correct "cpus per package".
> > 
> > When CPUState.nr_cores is correctly changed to "cores per package" again
> > , the above errors will be fixed without extra work, but the "currently"
> > correct cases will go wrong and need special handling to pass correct
> > "cpus/cores per die" they want.
> > 
> > Fix CPUState.nr_cores' calculation to fit the original meaning "cores
> > per package", as well as changing calculation of topo_info.cores_per_die,
> > vcpus_per_socket and CPUID.1FH.
> 
> What a pain. Can we split this patch in 2, first the x86 part
> and then the common part (softmmu/cpus.c)?

Since x86 uses this nr_cores to calculate many things, if the first
patch just fix nr_cores without changing x86 related code, then that
first patch will break many places (there will be many incorrect CPUIDs).

So I think it’s more appropriate to make these changes into one patch.

Thanks,
Zhao

> 
> > Fixes: a94e1428991f ("target/i386: Add CPUID.1F generation support for 
> > multi-dies PCMachine")
> > Fixes: 003f230e37d7 ("machine: Tweak the order of topology members in 
> > struct CpuTopology")
> > Signed-off-by: Zhuocheng Ding <zhuocheng.ding@intel.com>
> > Co-developed-by: Zhao Liu <zhao1.liu@intel.com>
> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > ---
> > Changes since v3:
> >   * Describe changes in imperative mood. (Babu)
> >   * Fix spelling typo. (Babu)
> >   * Split the comment change into a separate patch. (Xiaoyao)
> > 
> > Changes since v2:
> >   * Use wrapped helper to get cores per socket in qemu_init_vcpu().
> > 
> > Changes since v1:
> >   * Add comment for nr_dies in CPUX86State. (Yanan)
> > ---
> >   softmmu/cpus.c    | 2 +-
> >   target/i386/cpu.c | 9 ++++-----
> >   2 files changed, 5 insertions(+), 6 deletions(-)
> > 
> > diff --git a/softmmu/cpus.c b/softmmu/cpus.c
> > index 0848e0dbdb3f..fa8239c217ff 100644
> > --- a/softmmu/cpus.c
> > +++ b/softmmu/cpus.c
> > @@ -624,7 +624,7 @@ void qemu_init_vcpu(CPUState *cpu)
> >   {
> >       MachineState *ms = MACHINE(qdev_get_machine());
> > -    cpu->nr_cores = ms->smp.cores;
> > +    cpu->nr_cores = machine_topo_get_cores_per_socket(ms);
> >       cpu->nr_threads =  ms->smp.threads;
> >       cpu->stopped = true;
> >       cpu->random_seed = qemu_guest_random_seed_thread_part1();
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index 24ee67b42d05..709c055c8468 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -6015,7 +6015,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> > uint32_t count,
> >       X86CPUTopoInfo topo_info;
> >       topo_info.dies_per_pkg = env->nr_dies;
> > -    topo_info.cores_per_die = cs->nr_cores;
> > +    topo_info.cores_per_die = cs->nr_cores / env->nr_dies;
> >       topo_info.threads_per_core = cs->nr_threads;
> >       /* Calculate & apply limits for different index ranges */
> > @@ -6091,8 +6091,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> > uint32_t count,
> >                */
> >               if (*eax & 31) {
> >                   int host_vcpus_per_cache = 1 + ((*eax & 0x3FFC000) >> 14);
> > -                int vcpus_per_socket = env->nr_dies * cs->nr_cores *
> > -                                       cs->nr_threads;
> > +                int vcpus_per_socket = cs->nr_cores * cs->nr_threads;
> >                   if (cs->nr_cores > 1) {
> >                       *eax &= ~0xFC000000;
> >                       *eax |= (pow2ceil(cs->nr_cores) - 1) << 26;
> > @@ -6270,12 +6269,12 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t 
> > index, uint32_t count,
> >               break;
> >           case 1:
> >               *eax = apicid_die_offset(&topo_info);
> > -            *ebx = cs->nr_cores * cs->nr_threads;
> > +            *ebx = topo_info.cores_per_die * topo_info.threads_per_core;
> >               *ecx |= CPUID_TOPOLOGY_LEVEL_CORE;
> >               break;
> >           case 2:
> >               *eax = apicid_pkg_offset(&topo_info);
> > -            *ebx = env->nr_dies * cs->nr_cores * cs->nr_threads;
> > +            *ebx = cs->nr_cores * cs->nr_threads;
> >               *ecx |= CPUID_TOPOLOGY_LEVEL_DIE;
> >               break;
> >           default:
>