Bug 1298244 - kvm.conf - kvm_amd.nested=1
kvm.conf - kvm_amd.nested=1
Product: Fedora
Classification: Fedora
Component: qemu (Show other bugs)
Unspecified Unspecified
unspecified Severity unspecified
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Assigned To: Fedora Virtualization Maintainers
Fedora Extras Quality Assurance
: Reopened
Depends On:
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Reported: 2016-01-13 10:17 EST by poma
Modified: 2016-01-25 15:22 EST (History)
9 users (show)

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Doc Type: Bug Fix
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Last Closed: 2016-01-21 10:16:28 EST
Type: Bug
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Description poma 2016-01-13 10:17:21 EST
"add /etc/modprobe.d/kvm.conf"

#option kvm_amd nested=1

Do we need a reference to kvm_amd.nested=1 at all when:

"KVM: SVM: enable nested svm by default"

"Nested SVM is (in my experience) stable enough to be enabled by default. So omit the requirement to pass a module parameter."

static int nested = 1;


still is enabled per default:

static int nested = true;


# virsh dumpxml <domain> | grep cpu\ mode
  <cpu mode='host-passthrough'>

$ hostnamectl status 
   Static hostname: localhost.localdomain
         Icon name: computer-vm
           Chassis: vm
    Virtualization: qemu
  Operating System: Fedora 24 (Rawhide)
       CPE OS Name: cpe:/o:fedoraproject:fedora:24
            Kernel: Linux 4.4.0-1.fc24.x86_64
      Architecture: x86-64

$ cat /sys/module/kvm_amd/parameters/nested
Comment 1 poma 2016-01-13 10:20:47 EST
Perhaps -only- for kvm_intel.nested=1 because:

"KVM: nVMX: Add "nested" module option to kvm_intel"

"This patch adds to kvm_intel a module option "nested". This option controls whether the guest can use VMX instructions, i.e., whether we allow nested virtualization. A similar, but separate, option already exists for the SVM module. This option currently defaults to 0, meaning that nested VMX must be explicitly enabled by giving nested=1. When nested VMX matures, the default should probably be changed to enable nested VMX by default - just like nested SVM is currently enabled by default."


still is disabled per default:

static bool __read_mostly nested = 0;
Comment 2 poma 2016-01-13 10:25:23 EST
Care to comment, Mister Bonzini?
Comment 3 Cole Robinson 2016-01-13 10:27:38 EST
Your analysis is correct. amd defaults to nested=1, intel defaults to nested=0. But paolo indicated to me that the AMD default may be changes to nested=0 in the future. So I suggest we just wait and see
Comment 4 poma 2016-01-15 18:00:51 EST
Care to comment, Mister Bonzini?
Comment 5 Richard W.M. Jones 2016-01-16 03:18:57 EST
Paolo has already been asked about this, and the answer is
in comment 3.
Comment 6 poma 2016-01-16 04:05:47 EST
Care to comment, Mister Bonzini?
Comment 7 Cole Robinson 2016-01-21 10:16:28 EST
Here is the exact email paolo sent me:

On 01/10/2016 03:08 AM, Paolo Bonzini wrote:
> Hi Cole,
> The default is off for Intel and I am thinking of changing it for AMD, either upstream or in Rawhide. I haven't created a bug did the latter though.
> Paolo

If that's not sufficient for you, email him yourself. Please stop reopening this bug
Comment 8 Paolo Bonzini 2016-01-23 15:45:18 EST
Nested virtualization has never been audited for security, so I'll change it to disabled in 4.5.
Comment 9 poma 2016-01-25 15:22:19 EST
Yeah, from the production perspective that is the eventual problem.
After all that's your "bread and butter".

Good luck with that

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