Re: [PATCH v3] i386/cpu: fixup number of addressable IDs for logical processors in the physical package

[Igor Mammedov]

On Wed, 18 Sep 2024 21:18:15 +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
> 
> Try to execute "cpuid -1 -l 1 -r" in guest, we'll obtain a value of 90 for
> CPUID.01H.EBX[23:16], while the expected value is 128. And Try to
> execute "cpuid -1 -l 4 -r" in guest, we'll obtain a value of 63 for
> CPUID.04H.EAX[31:26] 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>

Reviewed-by: Igor Mammedov <imammedo@redhat.com>

> ---
>  target/i386/cpu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 4c2e6f3a71..3710ae5283 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -6417,7 +6417,7 @@ 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 |= pow2ceil(threads_per_pkg) << 16;
>              *edx |= CPUID_HT;
>          }
>          if (!cpu->enable_pmu) {