linux-kernel - Re: [PATCH] KVM: VMX: enable nested virtualization by default

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Message-ID: <CANRm+CyonYOzGdXo+D8gr8n04=f=S92QH-HxETKnoGGxhMFREA@mail.gmail.com>
Date:   Wed, 17 Oct 2018 08:44:47 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Liran Alon <liran.alon@...cle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] KVM: VMX: enable nested virtualization by default

On Wed, 17 Oct 2018 at 06:57, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> With live migration support and finally a good solution for CR2/DR6
> exception payloads, nested VMX should finally be ready for having a stable
> userspace ABI.  The results of syzkaller fuzzing are not perfect but not
> horrible either (and might be partially due to running on GCE, so that
> effectively we're testing three-level nesting on a fork of upstream KVM!).
> Enabling it by default seems like a nice way to conclude the 4.20
> pull request. :)
>
> Unfortunately, enabling nested SVM in 2009 was a bit premature.  However,
> until live migration support is in place we can reasonably expect that
> it does not offer much in terms of ABI guarantees.  Therefore we are
> still in time to break things and conform as much as possible to the
> interface used for VMX.
>
> Suggested-by: Jim Mattson <jmattson@...gle.com>
> Suggested-by: Liran Alon <liran.alon@...cle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/vmx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index e665aa7167cf..89fc2a744d7f 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -107,7 +107,7 @@ module_param_named(enable_shadow_vmcs, enable_shadow_vmcs, bool, S_IRUGO);
>   * VMX and be a hypervisor for its own guests. If nested=0, guests may not
>   * use VMX instructions.
>   */
> -static bool __read_mostly nested = 0;
> +static bool __read_mostly nested = 1;

Really cool, a milestone for nested. :)

Regards,
Wanpeng Li