Re: Xen HVM regression on certain Intel CPUs

["H. Peter Anvin" <hpa@zytor.com>]

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On 03/27/2013 09:24 AM, Stefan Bader wrote:
>> 
>> When you say Xen HVM, am I correct in assuming that neither CPUID
>> nor CR4 operations in the main kernel are run through
>> paravirt_ops?
> 
> Not paravirt ops likely but the hypervisor traps access. At least
> cpuid from within a hvm guest I expect to be filtered. So when
> checking things I went to bare-metal.
> 
> Will fetch more info and get back.
> 

Hypervisor traps is one thing... they should be consistent no matter
where in the code they happen... unless they are broken.

Try this CPUID program.  This uses the kernel /dev interface which may
be somewhat suboptimal in case CPUID in userspace actually differs,
but it would be interesting to know what it outputs.

	-hpa



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#define _FILE_OFFSET_BITS 64
#include <stdio.h>
#include <ctype.h>
#include <stdbool.h>
#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>

struct cpuid {
  uint32_t eax, ebx, ecx, edx;
};

static int cpuid(int cpu, uint32_t leaf, uint32_t subleaf, struct cpuid *data)
{
  static int fd = -1;
  static int last_cpu;
  off_t offset = leaf + ((off_t)subleaf << 32);

  if (fd < 0 || last_cpu != cpu) {
    char devstr[64];
    if (fd >= 0)
      close(fd);
    snprintf(devstr, sizeof devstr, "/dev/cpu/%d/cpuid", cpu);
    fd = open(devstr, O_RDONLY);
    last_cpu = cpu;
  }
  return pread(fd, data, sizeof(*data), offset) == sizeof(*data) ? 0 : -1;
}

static char *make_string(uint32_t val)
{
  static char string[5] = "xxxx";
  int i, ch;

  for ( i = 0 ; i < 4 ; i++ ) {
    ch = val & 0xff;
    string[i] = isprint(ch) ? ch : '.';
    val >>= 8;
  }

  return string;
}

static void print_cpuid_level(uint32_t leaf, uint32_t subleaf,
			      struct cpuid *lvl)
{
  printf("%08x %08x:  ", leaf, subleaf);
  printf("%08x %s  ", lvl->eax, make_string(lvl->eax));
  printf("%08x %s  ", lvl->ebx, make_string(lvl->ebx));
  printf("%08x %s  ", lvl->ecx, make_string(lvl->ecx));
  printf("%08x %s\n", lvl->edx, make_string(lvl->edx));
}

static void dump_cpuid_leaf(int cpu, uint32_t leaf)
{
  struct cpuid lvl, lastlvl, lvl0;
  uint32_t subleaf;

  cpuid(cpu, leaf, 0, &lvl0);
  print_cpuid_level(leaf, 0, &lvl0);

  /*
   * There is no standard mechanism for enumerating the number of
   * subleaves, this is a heuristic...
   */
  lastlvl = lvl0;

  for (subleaf = 1; subleaf != 0; subleaf++) {
    if (cpuid(cpu, leaf, subleaf, &lvl))
      return;

    switch (leaf) {
    case 4:
      if ((lvl.eax & 0x1f) == 0 || !memcmp(&lvl, &lastlvl, sizeof lvl))
	return;
      break;

    case 7:
      if (subleaf >= lvl0.eax)
	return;
      break;

    case 0xb:
      if ((lvl.ecx & ~0xff) == 0)
	return;

    case 0xd:
      if ((lvl.eax | lvl.ebx | lvl.ecx | lvl.edx) == 0)
	return;

    default:
      /* Generic, anticipatory rules */
      /* Exclude ecx here for levels which return the initial ecx value */
      if ((lvl.eax | lvl.ebx | lvl.ecx | lvl.edx) == 0)
	return;
    
      if (!memcmp(&lvl, &lvl0, sizeof lvl))
	return;
      break;
    }
    
    print_cpuid_level(leaf, subleaf, &lvl);

    lastlvl = lvl;
  }
}

static void dump_levels(int cpu, uint32_t region)
{
  static struct cpuid invalid_leaf;
  struct cpuid max;
  uint32_t n;

  if (cpuid(cpu, region, 0, &max))
    return;

  /*
   * Intel processors may return the last group 0 CPUID leaf instead
   * all zero for a not-present level
   */
  if (region == 0) {
    cpuid(cpu, max.eax+1, 0, &invalid_leaf);
  } else {
    if (!memcmp(&max, &invalid_leaf, sizeof(struct cpuid)))
      return;
  }

  if ( (max.eax & 0xffff0000) == region ) {
    for ( n = region ; n <= max.eax ; n++ ) {
      dump_cpuid_leaf(cpu, n);
    }
  }
}

int main(int argc, char *argv[])
{
  int cpu;
  uint32_t n;

  cpu = (argc > 1) ? atoi(argv[1]) : 0;
  
  printf("Leaf     Subleaf    EAX            EBX            ECX            EDX            \n");

  for ( n = 0 ; n <= 0xffff ; n++ ) {
    dump_levels(cpu, n << 16);
  }

  return 0;
}

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