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From: bugzilla-daemon@bugzilla.kernel.org
Subject: [Bug 77271] New: CPUID Leaf 0x40000000 return 0 in eax
Date: Tue, 03 Jun 2014 19:48:05 +0000
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https://bugzilla.kernel.org/show_bug.cgi?id=77271

            Bug ID: 77271
           Summary: CPUID Leaf 0x40000000 return 0 in eax
           Product: Virtualization
           Version: unspecified
    Kernel Version: 3.14
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: kvm
          Assignee: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: jidong.xiao@gmail.com
        Regression: No

I am using kernel 3.14 as the host OS. And using 2.6.34 as the guest OS kernel.

According to 
https://github.com/torvalds/linux/commit/57c22e5f35aa4b9b2fe11f73f3e62bbf9ef36190

Since kernel 3.5, the cpuid instruction (in the Guest OS) with 0x40000000
function should return 0x40000001 in eax, instead of 0. But what I am seeing
is, it still returns 0.

To reproduce, use a program like this, run it in the Guest:
linux:~/code # cat cpusig.c
#include <stdio.h>

#define cpuid(func,eax,ebx,ecx,edx)\
        __asm__ __volatile__ ("cpuid":\
        "=a" (eax), "=b" (ebx), "=c" (ecx), "=d" (edx) : "a" (func));

main()
{
        unsigned long a,b,c,d;
        cpuid(0x40000000,a,b,c,d);
        printf("0x40000000 KVM_CPUID_SIGNATURE %lx %lx %lx %lx\n",a,b,c,d);
}

What I saw is:
linux:~/code # ./cpusig
0x40000000 KVM_CPUID_SIGNATURE 0 4b4d564b 564b4d56 4d

It looks like ebx,ecx,edx all contain right values, but eax does not.

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