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

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Date:   Wed, 17 Oct 2018 02:13:15 +0300
From:   Liran Alon <liran.alon@...cle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        jmattson@...gle.com
Subject: Re: [PATCH] KVM: VMX: enable nested virtualization by default



> On 17 Oct 2018, at 1:55, 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

And good solution for setting/getting vCPU events from userspace with correct pending/injected state.

> 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,

Don’t you wish to mention commit which enabled it?

> 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;
> module_param(nested, bool, S_IRUGO);
> 
> static u64 __read_mostly host_xss;
> -- 
> 2.17.1
> 

Woohoo! :)
Reviewed-by: Liran Alon <liran.alon@...cle.com>