Re: [PATCH v6] i386/cpu: fixup number of addressable IDs for logical processors in the physical package

[Xiaoyao Li]

On 10/9/2024 11:56 AM, Chuang Xu wrote:
> When QEMU is started with:
> -cpu host,migratable=on,host-cache-info=on,l3-cache=off
> -smp 180,sockets=2,dies=1,cores=45,threads=2
>
> On Intel platform:
> CPUID.01H.EBX[23:16] is defined as "max number of addressable IDs for
> logical processors in the physical package".
>
> When executing "cpuid -1 -l 1 -r" in the guest, we obtain a value of 90 for
> CPUID.01H.EBX[23:16], whereas the expected value is 128. Additionally,
> executing "cpuid -1 -l 4 -r" in the guest yields a value of 63 for
> CPUID.04H.EAX[31:26], which matches the expected result.
>
> As (1+CPUID.04H.EAX[31:26]) rounds up to the nearest power-of-2 integer,
> we'd beter round up CPUID.01H.EBX[23:16] to the nearest power-of-2
> integer too. Otherwise we may encounter unexpected results in guest.
>
> For example, when QEMU is started with CLI above and xtopology is disabled,
> guest kernel 5.15.120 uses CPUID.01H.EBX[23:16]/(1+CPUID.04H.EAX[31:26]) to
> calculate threads-per-core in detect_ht(). Then guest will get "90/(1+63)=1"
> as the result, even though threads-per-core should actually be 2.

It's kernel's bug instead.

In 1.5.3 "Sub ID Extraction Parameters for initial APIC ID" of "Intel 64 
Architecture Processor Topology Enumeration" [1], it is

  - SMT_Mask_Width = Log2(RoundToNearestPof2(CPUID.1:EBX[23:16])/ 
(CPUID.(EAX=4,ECX=0):EAX[31:26]) + 1))

The value of CPUID.1:EBX[23:16] needs to be *rounded* to the neartest 
power-of-two integer instead of itself being the power-of-two.

This also is consistency with the SDM, where the comment for bit 23-16 
of CPUID.1:EBX is:

  The nearest power-of-2 integer that is not smaller than EBX[23:16] is
  the number of unique initial APIC IDs reserved for addressing
  different logical processors in a physical package.

What I read from this is, the nearest power-of-2 integer that is not 
smaller than EBX[23:16] is a different thing than EBX[23:16]. i.e.,

- EBX[23:16]: Maximum number of addressable IDs for logical processors
  in this physical package

- pow2ceil(EBX[23:16]): the number of unique initial APIC IDs reserved
  for addressing different logical processors in a physical package.

[1] 
https://cdrdv2-public.intel.com/759067/intel-64-architecture-processor-topology-enumeration.pdf

> And on AMD platform:
> CPUID.01H.EBX[23:16] is defined as "Logical processor count". Current
> result meets our expectation.
>
> So let us round up CPUID.01H.EBX[23:16] to the nearest power-of-2 integer
> only for Intel platform to solve the unexpected result.
>
> Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
> Acked-by: Igor Mammedov <imammedo@redhat.com>
> Signed-off-by: Guixiong Wei <weiguixiong@bytedance.com>
> Signed-off-by: Yipeng Yin <yinyipeng@bytedance.com>
> Signed-off-by: Chuang Xu <xuchuangxclwt@bytedance.com>
> ---
>   target/i386/cpu.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index ff227a8c5c..641d4577b0 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -6462,7 +6462,15 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> uint32_t count,
>           }
>           *edx = env->features[FEAT_1_EDX];
>           if (threads_per_pkg > 1) {
> -            *ebx |= threads_per_pkg << 16;
> +            /*
> +             * AMD requires logical processor count, but Intel needs maximum
> +             * number of addressable IDs for logical processors per package.
> +             */
> +            if (cpu->vendor_cpuid_only && IS_AMD_CPU(env)) {
> +                *ebx |= threads_per_pkg << 16;
> +            } else {
> +                *ebx |= 1 << apicid_pkg_offset(&topo_info) << 16;
> +            }
>               *edx |= CPUID_HT;
>           }
>           if (!cpu->enable_pmu) {