On 8/7/2023 3:36 PM, Xiaoyao Li wrote:
> On 7/28/2023 4:01 PM, Qian Wen 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.
>>
>> To avoid data overflow, limit the max value written to EBX[23:16] to
>> 255.
>
> It's better explain what's issue when overflow happens.
>
When launch vm with -smp 256, the value writes to EBX[23:16] is 0.
If the guest only support legacy topology, the result of kernel invokes
cpu_smt_allowed() is false and AP's bring-up will fail. Then only CPU 0 is
online, others offline.
>> Signed-off-by: Qian Wen <qian....@intel.com>
>> ---
>> target/i386/cpu.c | 15 +++++++++++++--
>> 1 file changed, 13 insertions(+), 2 deletions(-)
>>
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index 1294be374ab2..70589a58b727 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -5356,6 +5356,7 @@ 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;
>> @@ -5397,8 +5398,18 @@ 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;
>> + /*
>> + * The vCPU number more than 255 needs support of V2 Extended
>> + * Topology enumerated by CPUID.0x1f or Extended Topology
>> + * enumerated by CPUID.0x0b.
>> + */
>
> the above comment doesn't explain why it needs below.
>
> you can explain only bits [23:16] represents 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] gets truncated.
Thanks for your suggestion, I will add your description in v2.
Thanks,
Qian
>
>> + 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;
>> }
>> /*
>
