[Bug 77271] New: CPUID Leaf 0x40000000 return 0 in eax

[Bug 77271] New: CPUID Leaf 0x40000000 return 0 in eax

[bugzilla-daemon]

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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[Bug 77271] CPUID Leaf 0x40000000 return 0 in eax

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=77271

Alex Williamson <alex.williamson@redhat.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alex.williamson@redhat.com

--- Comment #1 from Alex Williamson <alex.williamson@redhat.com> ---
Further up in cpuid.txt is the statement:

"This is not always guaranteed to work, since userspace can
mask-out some, or even all KVM-related cpuid features before launching
a guest."

Which is exactly what is happening here.  QEMU sets eax = 0 for this leaf:

target-i386/kvm.c
    memcpy(signature, "KVMKVMKVM\0\0\0", 12);
    c = &cpuid_data.entries[cpuid_i++];
    c->function = KVM_CPUID_SIGNATURE | kvm_base;
    c->eax = 0;
    c->ebx = signature[0];
    c->ecx = signature[1];
    c->edx = signature[2];

So I don't think there's a bug here.

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[Bug 77271] CPUID Leaf 0x40000000 return 0 in eax

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=77271

--- Comment #2 from Jidong Xiao <jidong.xiao@gmail.com> ---
Oh, thanks Alex. I think you are right here, it's not a bug. But do you think
that, does it make sense to submit a patch to qemu, so as to make them
consistent between userspace qemu and the kernel side kvm? Like this:

diff --git a/qemu-2.0.0/target-i386/kvm.c.orig b/qemu-2.0.0/target-i386/kvm.c
index 4389959..b8b282d 100644
--- a/qemu-2.0.0/target-i386/kvm.c.orig
+++ b/qemu-2.0.0/target-i386/kvm.c
@@ -530,7 +530,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
     memcpy(signature, "KVMKVMKVM\0\0\0", 12);
     c = &cpuid_data.entries[cpuid_i++];
     c->function = KVM_CPUID_SIGNATURE | kvm_base;
-    c->eax = 0;
+    c->eax = KVM_CPUID_FEATURES;
     c->ebx = signature[0];
     c->ecx = signature[1];
     c->edx = signature[2];

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[Bug 77271] CPUID Leaf 0x40000000 return 0 in eax

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https://bugzilla.kernel.org/show_bug.cgi?id=77271

--- Comment #3 from Alex Williamson <alex.williamson@redhat.com> ---
The best way to find out is to submit a patch.

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[Bug 77271] CPUID Leaf 0x40000000 return 0 in eax

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=77271

Jidong Xiao <jidong.xiao@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #4 from Jidong Xiao <jidong.xiao@gmail.com> ---
Great, thank Alex, I have submitted a patch to Qemu, I think this ticket can be
closed now.

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