Re: [PATCH v2] target/i386: Avoid cpu number overflow in legacy topology

["Wen, Qian" <qian.wen@intel.com>]

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On 8/9/2023 7:14 PM, Igor Mammedov wrote:
> On Wed,  9 Aug 2023 18:27:32 +0800
> Qian Wen <qian.wen@intel.com> wrote:
>
>> The legacy topology enumerated by CPUID.1.EBX[23:16] is defined in SDM
>> Vol2:
>>
>> Bits 23-16: Maximum number of addressable IDs for logical processors in
>> this physical package.
>>
>> When launching the VM with -smp 256, the value written to EBX[23:16] is
>> 0 because of data overflow. If the guest only supports legacy topology,
>> without V2 Extended Topology enumerated by CPUID.0x1f or Extended
>> Topology enumerated by CPUID.0x0b to support over 255 CPUs, the return
>> of the kernel invoking cpu_smt_allowed() is false and AP's bring-up will
>> fail. Then only CPU 0 is online, and others are offline.
>>
>> To avoid this issue caused by overflow, limit the max value written to
>> EBX[23:16] to 255.
> what happens on real hw or in lack of thereof what SDM says about this
> value when there is more than 255 threads?.
>

Current SDM doesn't specify what the value should be when APIC IDs per package exceeds 255. So we asked the internal HW architect, the response is that EBX[23:16] will report 255 instead of being truncated to a smaller value.

Thanks,
Qian

>> Signed-off-by: Qian Wen <qian.wen@intel.com>
>> ---
>> Changes v1 -> v2:
>>  - Revise the commit message and comment to more clearer.
>>  - Rebased to v8.1.0-rc2.
>> ---
>>  target/i386/cpu.c | 16 ++++++++++++++--
>>  1 file changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index 97ad229d8b..6e1d88fbd7 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -6008,6 +6008,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
>>      uint32_t die_offset;
>>      uint32_t limit;
>>      uint32_t signature[3];
>> +    uint32_t threads_per_socket;
>>      X86CPUTopoInfo topo_info;
>>  
>>      topo_info.dies_per_pkg = env->nr_dies;
>> @@ -6049,8 +6050,19 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
>>              *ecx |= CPUID_EXT_OSXSAVE;
>>          }
>>          *edx = env->features[FEAT_1_EDX];
>> -        if (cs->nr_cores * cs->nr_threads > 1) {
>> -            *ebx |= (cs->nr_cores * cs->nr_threads) << 16;
>> +        /*
>> +         * Only bits [23:16] represent the maximum number of addressable
>> +         * IDs for logical processors in this physical package.
>> +         * When thread_per_socket > 255, it will 1) overwrite bits[31:24]
>> +         * which is apic_id, 2) bits [23:16] get truncated.
>> +         */
>> +        threads_per_socket = cs->nr_cores * cs->nr_threads;
>> +        if (threads_per_socket > 255) {
>> +            threads_per_socket = 255;
>> +        }
>> +
>> +        if (threads_per_socket > 1) {
>> +            *ebx |= threads_per_socket << 16;
>>              *edx |= CPUID_HT;
>>          }
>>          if (!cpu->enable_pmu) {

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