Re: [PATCH v2] i386/cpu: fixup number of addressable IDs for logical processors in the physical package

[Chuang Xu]

Hi, Igor:

On 2024/9/18 下午8:02, Igor Mammedov wrote:
> On Sat, 14 Sep 2024 19:01:27 +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
> >
> > Execute "cpuid -1 -l 1 -r" in guest, we'll get:
> > eax=0x000806f8 ebx=0x465a0800 ecx=0xfffaba1f edx=0x3fa9fbff
> > CPUID.01H.EBX[23:16] is 90, while the expected value is 128.
> >
> > Execute "cpuid -1 -l 4 -r" in guest, we'll get:
> > eax=0xfc004121 ebx=0x02c0003f ecx=0x0000003f edx=0x00000000
> > CPUID.04H.EAX[31:26] is 63, which is 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 | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index 4c2e6f3a71..24d60ead9e 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -261,6 +261,12 @@ static uint32_t max_thread_ids_for_cache(X86CPUTopoInfo 
> > *topo_info,
> >       return num_ids - 1;
> >   }
> >   
> > +static uint32_t max_thread_number_in_package(X86CPUTopoInfo *topo_info)
> > +{
> > +    uint32_t num_threads = 1 << apicid_pkg_offset(topo_info);
> > +    return num_threads;
> > +}
> > +
> >   static uint32_t max_core_ids_in_package(X86CPUTopoInfo *topo_info)
> >   {
> >       uint32_t num_cores = 1 << (apicid_pkg_offset(topo_info) -
> > @@ -6417,7 +6423,7 @@ 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 |= max_thread_number_in_package(&topo_info) << 16;
> why not use pow2ceil(threads_per_pkg) instead?

I saw in the latest code that calculations of cpuids involving CPU topology all 
use topo_info,
so in order to maintain consistency in code style, I also used topo_info for 
calculation.

> >               *edx |= CPUID_HT;
> >           }
> >           if (!cpu->enable_pmu) {