On 8/9/2023 7:14 PM, Igor Mammedov wrote:
> On Wed, 9 Aug 2023 18:27:32 +0800
> Qian Wen <qian....@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....@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) {
