Re: [PATCH v2] i386/cpu: fixup number of addressable IDs for logical processors in the physical package

[Igor Mammedov]

On Sat, 14 Sep 2024 19:01:27 +0800
Chuang Xu <xuchuangxclwt@bytedance.com> 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
> 
> Execute "cpuid -1 -l 1 -r" in guest, we'll get:
> eax=0x000806f8 ebx=0x465a0800 ecx=0xfffaba1f edx=0x3fa9fbff
> CPUID.01H.EBX[23:16] is 90, while the expected value is 128.
> 
> Execute "cpuid -1 -l 4 -r" in guest, we'll get:
> eax=0xfc004121 ebx=0x02c0003f ecx=0x0000003f edx=0x00000000
> CPUID.04H.EAX[31:26] is 63, which is as expected.
> 
> As (1+CPUID.04H.EAX[31:26]) round 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 theads-per-core should actually be 2.
> 
> So let us round up CPUID.01H.EBX[23:16] to the nearest power-of-2 integer
> to solve the unexpected result.
> 
> 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 | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 4c2e6f3a71..24d60ead9e 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -261,6 +261,12 @@ static uint32_t max_thread_ids_for_cache(X86CPUTopoInfo 
> *topo_info,
>      return num_ids - 1;
>  }
>  
> +static uint32_t max_thread_number_in_package(X86CPUTopoInfo *topo_info)
> +{
> +    uint32_t num_threads = 1 << apicid_pkg_offset(topo_info);
> +    return num_threads;
> +}
> +
>  static uint32_t max_core_ids_in_package(X86CPUTopoInfo *topo_info)
>  {
>      uint32_t num_cores = 1 << (apicid_pkg_offset(topo_info) -
> @@ -6417,7 +6423,7 @@ 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;
> +            *ebx |= max_thread_number_in_package(&topo_info) << 16;

why not use pow2ceil(threads_per_pkg) instead?

>              *edx |= CPUID_HT;
>          }
>          if (!cpu->enable_pmu) {