diff mbox series

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

Message ID 20240914110127.67690-1-xuchuangxclwt@bytedance.com (mailing list archive)
State New, archived
Headers show
Series [v2] i386/cpu: fixup number of addressable IDs for logical processors in the physical package | expand

Commit Message

Chuang Xu Sept. 14, 2024, 11:01 a.m. UTC 
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(-)

Comments

Igor Mammedov Sept. 18, 2024, 12:02 p.m. UTC | #1 
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?

>              *edx |= CPUID_HT;
>          }
>          if (!cpu->enable_pmu) {
Chuang Xu Sept. 18, 2024, 12:22 p.m. UTC | #2 
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) {
Igor Mammedov Sept. 18, 2024, 12:43 p.m. UTC | #3 
On Wed, 18 Sep 2024 20:22:26 +0800
Chuang Xu <xuchuangxclwt@bytedance.com> wrote:

> 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.

and we end up with a zoo of ways different topo stuff is calculated.

Given we already have threads_per_pkg calculated within the function,
is cleaner/more self-documenting to reuse it with pow2ceil() instead of
adding yet another helper with less than obvious '1 << apicid_pkg_offset(topo_info)' math.

> >  
> >>               *edx |= CPUID_HT;
> >>           }
> >>           if (!cpu->enable_pmu) 
>
diff mbox series

Patch

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