Difference L2 size between CPUID(4) and CPUID(80000006h)

DParks · ‎04-25-2019

It is not obvious from the descriptions of the L2 cache sizes using CPUID(4) and CPUID(80000006h) in the software developer's manual volume 2 (325383-063US), why the two inquiry methods would return different sizes.

CPUID(80000006h) returns (eax, ebx, ecx, edx): 0 0 0x1006040 0. Documentation says that bits 31:16 of ecx is L2 cache size in K units, which translates to 256K.

The L2 cache information from CPUID(4) returns (eax, ebx, ecx, edx): 0x7c004143 0x3c0003f 0x3ff 0, which according to (heading page 3-216 Vol 2A.) "INPUT EAX = 04H: Returns Deterministic Cache Parameters for Each Level" size is computed as:

This Cache Size in Bytes

= (Ways + 1) * (Partitions + 1) * (Line_Size + 1) * (Sets + 1)

= (EBX[31:22] + 1) * (EBX[21:12] + 1) * (EBX[11:0] + 1) * (ECX + 1)

Which I believe given the ebx/ecx values comes to 1048576.

/proc/cpuinfo (Linux) shows:

processor : 0

vendor_id : GenuineIntel

cpu family : 6

model : 85

model name : Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz

stepping : 4

microcode : 0x200004d

cpu MHz : 1000.000

My question is: which method is correct? The 1MB L2 cache size as returned by CPUID(4) seems to match with all public information for this processor/model.

BTW, the discrepancy in L2 sizes only seems to happen with the Skylake Xeon processor that I have access to.

Dave

Here's my driver to the CPUID calls:

#include <stdio.h>
#include <stdint.h>
int cpuid_ecx(uint32_t eax, uint32_t *res, uint32_t ecx)
{
  asm("cpuid"
  : "=a"(res[0]), "=b"(res[1]), "=c"(res[2]), "=d"(res[3])
  : "a"(eax), "c"(ecx)
  :);
}
 
int
main()
{
  uint32_t r[4];
  uint32_t ecx = 0;
  uint32_t csz;
 
  cpuid_ecx(0x80000006, r, ecx);
  printf("%u %#x %#x %#x %#x\n", ecx, r[0], r[1], r[2], r[3]);
  printf("0x80000006 L2 size=%u\n", r[2] >> 16);
 
  while (1) {
    cpuid_ecx(0x4, r, ecx);
    printf("%u %#x %#x %#x %#x\n", ecx, r[0], r[1], r[2], r[3]);
    if (r[0] == 0) break;
    csz = (r[1] >> 22) + 1;
    csz *= ((r[1] >> 12) & 0x3ff) + 1;
    csz *= (r[1] & 0x3ff) + 1;
    csz *= r[2] + 1;
    printf("L%u size = %u\n", (r[0] >> 5) & 0x3, csz);
    ++ecx;
  }
}

Oscar_A_Intel1 · ‎04-26-2019

Hello DParks, Thank you for posting on the Intel Community. We provide basic troubleshooting for processors, mini PCs, etc.. For this type of inquiries please go to the Intel® Developer Zone for the appropriate support. https://software.intel.com/en-us/home Intel® Developer Zone forums: https://software.intel.com/en-us/forum Regards, Oscar A. Intel Customer Support Technician Under Contract to Intel Corporation