Re: [PATCH v3 04/17] i386/cpu: Fix i/d-cache topology to core level for Intel CPU

[Xiaoyao Li]

On 8/1/2023 6:35 PM, Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@intel.com>
>
> For i-cache and d-cache, the maximum IDs for CPUs sharing cache (
> CPUID.04H.00H:EAX[bits 25:14] and CPUID.04H.01H:EAX[bits 25:14]) are
> both 0, 

This sounds like you are describing some architectural rules, which 
misleads me. I suggest changing the description to

For i-cache and d-cache, current QEMU hardcodes the maximum IDs for CPUs 
sharing cache (CPUID.04H.00H:EAX[bits 25:14] and CPUID.04H.01H:EAX[bits 
25:14]) to 0. ...

> and this means i-cache and d-cache are shared in the SMT level.
> This is correct if there's single thread per core, but is wrong for the
> hyper threading case (one core contains multiple threads) since the
> i-cache and d-cache are shared in the core level other than SMT level.
>
> For AMD CPU, commit 8f4202fb1080 ("i386: Populate AMD Processor Cache
> Information for cpuid 0x8000001D") has already introduced i/d cache
> topology as core level by default.
>
> Therefore, in order to be compatible with both multi-threaded and
> single-threaded situations, we should set i-cache and d-cache be shared
> at the core level by default.
>
> This fix changes the default i/d cache topology from per-thread to
> per-core. Potentially, this change in L1 cache topology may affect the
> performance of the VM if the user does not specifically specify the
> topology or bind the vCPU. However, the way to achieve optimal
> performance should be to create a reasonable topology and set the
> appropriate vCPU affinity without relying on QEMU's default topology
> structure.
>
> Fixes: 7e3482f82480 ("i386: Helpers to encode cache information consistently")
> Suggested-by: Robert Hoo <robert.hu@linux.intel.com>
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

> ---
> Changes since v1:
>   * Split this fix from the patch named "i386/cpu: Fix number of
>     addressable IDs in CPUID.04H".
>   * Add the explanation of the impact on performance. (Xiaoyao)
> ---
>   target/i386/cpu.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 50613cd04612..b439a05244ee 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -6104,12 +6104,12 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> uint32_t count,
>               switch (count) {
>               case 0: /* L1 dcache info */
>                   encode_cache_cpuid4(env->cache_info_cpuid4.l1d_cache,
> -                                    1, cs->nr_cores,
> +                                    cs->nr_threads, cs->nr_cores,
>                                       eax, ebx, ecx, edx);
>                   break;
>               case 1: /* L1 icache info */
>                   encode_cache_cpuid4(env->cache_info_cpuid4.l1i_cache,
> -                                    1, cs->nr_cores,
> +                                    cs->nr_threads, cs->nr_cores,
>                                       eax, ebx, ecx, edx);
>                   break;
>               case 2: /* L2 cache info */