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

[Zhao Liu]

Hi Chuang,

Could I pick this fix in my later series (with another overflow fix)?
I can help you push this fix forward :-).

Regards,
Zhao

On Wed, Oct 09, 2024 at 11:56:38AM +0800, Chuang Xu wrote:
> Date: Wed,  9 Oct 2024 11:56:38 +0800
> From: Chuang Xu <xuchuangxclwt@bytedance.com>
> Subject: [PATCH v6] i386/cpu: fixup number of addressable IDs for logical
>  processors in the physical package
> X-Mailer: git-send-email 2.39.3 (Apple Git-146)
> 
> 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.
> 
> 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>