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

[Zhao Liu]

Hi Chuang and Igor,

Sorry for late reply,

On Wed, Sep 18, 2024 at 09:18:15PM +0800, Chuang Xu wrote:
> Date: Wed, 18 Sep 2024 21:18:15 +0800
> From: Chuang Xu <xuchuangxclwt@bytedance.com>
> Subject: [PATCH v3] i386/cpu: fixup number of addressable IDs for logical
>  processors in the physical package
> X-Mailer: git-send-email 2.24.3 (Apple Git-128)
> 
> 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>
> ---
>  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;

Yes, the fix is right.

About the "Maximum number of addressable IDs", the commit 88dd4ca06c83
("i386/cpu: Use APIC ID info to encode cache topo in CPUID[4]")
introduced the new way to calculate.

The pow2ceil() works for current SMP topology, but may be wrong on
hybrid topology, as the reason I listed in the commit message:

> The nearest power-of-2 integer can be calculated by pow2ceil() or by
> using APIC ID offset/width (like L3 topology using 1 << die_offset [3]).

> But in fact, CPUID.04H:EAX[bits 25:14] and CPUID.04H:EAX[bits 31:26]
> are associated with APIC ID. For example, in linux kernel, the field
> "num_threads_sharing" (Bits 25 - 14) is parsed with APIC ID. And for
> another example, on Alder Lake P, the CPUID.04H:EAX[bits 31:26] is not
> matched with actual core numbers and it's calculated by:
> "(1 << (pkg_offset - core_offset)) - 1".

Using APIC ID offset to calculate is the hardware's approach, so I tried
to use APIC ID instead of pow2ceil() and replaced all pow2ceil() case.

Hi Igor, do you agree? :-)

Best Regards,
Zhao