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Date: Fri, 1 Aug 2014 21:47:05 +0100
From: "Richard W.M. Jones" <rjones@redhat.com>
Message-ID: <20140801204705.GC14001@redhat.com>
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Subject: Re: [Qemu-devel] about -enable-kvm options
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To: qemu-devel@nongnu.org

On Fri, Aug 01, 2014 at 11:15:29AM +0800, Gareth wrote:
> Hi all
> 
> What does '-enable-kvm' option mean? I have heard two versions of answers:

It's a shortcut for:

  $qemu -machine accel=kvm

> a) guest OS would have /dev/kvm device and which could help vm in guest OS
> (nested vm)

That's nested KVM, which is enabled as a module option in the kernel
module (on the host), eg:

  # modprobe kvm_intel nested=1

On AMD it's enabled by default.

> b) use /dev/kvm and intel-vt on host OS which could help vm run more fast
> than pure emulator.

Nearly.  It uses /dev/kvm on the host, which may or may not be
implemented using Intel VT, or a variety of other techniques.

You can also have fallbacks to software emulation (TCG):

  $qemu -machine accel=kvm:tcg

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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