Re: [PATCH v7 15/16] i386: Use offsets get NumSharingCache for CPUID[0x8000001D].EAX[bits 25:14]

[Zhao Liu]

Hi Xiaoyao,

On Sun, Jan 14, 2024 at 10:42:41PM +0800, Xiaoyao Li wrote:
> Date: Sun, 14 Jan 2024 22:42:41 +0800
> From: Xiaoyao Li <xiaoyao.li@intel.com>
> Subject: Re: [PATCH v7 15/16] i386: Use offsets get NumSharingCache for
>  CPUID[0x8000001D].EAX[bits 25:14]
> 
> On 1/8/2024 4:27 PM, Zhao Liu wrote:
> > From: Zhao Liu <zhao1.liu@intel.com>
> > 
> > The commit 8f4202fb1080 ("i386: Populate AMD Processor Cache Information
> > for cpuid 0x8000001D") adds the cache topology for AMD CPU by encoding
> > the number of sharing threads directly.
> > 
> >  From AMD's APM, NumSharingCache (CPUID[0x8000001D].EAX[bits 25:14])
> > means [1]:
> > 
> > The number of logical processors sharing this cache is the value of
> > this field incremented by 1. To determine which logical processors are
> > sharing a cache, determine a Share Id for each processor as follows:
> > 
> > ShareId = LocalApicId >> log2(NumSharingCache+1)
> > 
> > Logical processors with the same ShareId then share a cache. If
> > NumSharingCache+1 is not a power of two, round it up to the next power
> > of two.
> > 
> >  From the description above, the calculation of this field should be same
> > as CPUID[4].EAX[bits 25:14] for Intel CPUs. So also use the offsets of
> > APIC ID to calculate this field.
> > 
> > [1]: APM, vol.3, appendix.E.4.15 Function 8000_001Dh--Cache Topology
> >       Information
> 
> this patch can be dropped because we have next patch.

This patch is mainly used to explicitly emphasize the change in encoding
way and compliance with AMD spec... I didn't tested on AMD machine, so
the more granular patch would make it easier for the community to review
and test.

Thanks,
Zhao

> 
> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > Reviewed-by: Babu Moger <babu.moger@amd.com>
> > Tested-by: Babu Moger <babu.moger@amd.com>
> > Tested-by: Yongwei Ma <yongwei.ma@intel.com>
> > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > Changes since v3:
> >   * Rewrite the subject. (Babu)
> >   * Delete the original "comment/help" expression, as this behavior is
> >     confirmed for AMD CPUs. (Babu)
> >   * Rename "num_apic_ids" (v3) to "num_sharing_cache" to match spec
> >     definition. (Babu)
> > 
> > Changes since v1:
> >   * Rename "l3_threads" to "num_apic_ids" in
> >     encode_cache_cpuid8000001d(). (Yanan)
> >   * Add the description of the original commit and add Cc.
> > ---
> >   target/i386/cpu.c | 10 ++++------
> >   1 file changed, 4 insertions(+), 6 deletions(-)
> > 
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index b23e8190dc68..8a4d72f6f760 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -483,7 +483,7 @@ static void encode_cache_cpuid8000001d(CPUCacheInfo 
> > *cache,
> >                                          uint32_t *eax, uint32_t *ebx,
> >                                          uint32_t *ecx, uint32_t *edx)
> >   {
> > -    uint32_t l3_threads;
> > +    uint32_t num_sharing_cache;
> >       assert(cache->size == cache->line_size * cache->associativity *
> >                             cache->partitions * cache->sets);
> > @@ -492,13 +492,11 @@ static void encode_cache_cpuid8000001d(CPUCacheInfo 
> > *cache,
> >       /* L3 is shared among multiple cores */
> >       if (cache->level == 3) {
> > -        l3_threads = topo_info->modules_per_die *
> > -                     topo_info->cores_per_module *
> > -                     topo_info->threads_per_core;
> > -        *eax |= (l3_threads - 1) << 14;
> > +        num_sharing_cache = 1 << apicid_die_offset(topo_info);
> >       } else {
> > -        *eax |= ((topo_info->threads_per_core - 1) << 14);
> > +        num_sharing_cache = 1 << apicid_core_offset(topo_info);
> >       }
> > +    *eax |= (num_sharing_cache - 1) << 14;
> >       assert(cache->line_size > 0);
> >       assert(cache->partitions > 0);
>