Re: [RFC PATCH v2 1/3] KVM: x86: only allow exits disable before vCPUs created

[Sean Christopherson <seanjc@google.com>]

On Tue, Dec 21, 2021, Kechen Lu wrote:
> Since VMX and SVM both would never update the control bits if exits
> are disable after vCPUs are created, only allow setting exits
> disable flag before vCPU creation.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>

For this to carry my SOB, I should be attributed as the author, or add a
Co-developed-by: for me.  I'm also totally ok with a Suggested-by: or Reported-by:

And we should at least have

  Fixes: 4d5422cea3b6 ("KVM: X86: Provide a capability to disable MWAIT intercepts")

andy maybe Cc: stable@vger.kernel.org, though I'm not entirely sure this is stable
material as it could in theory do more harm than good if there's a busted userspace
out there.

If this doesn't carry my SOB...

Reviewed-by: Sean Christopherson <seanjc@google.com>

> Signed-off-by: Kechen Lu <kechenl@nvidia.com>
> ---
>  Documentation/virt/kvm/api.rst | 1 +
>  arch/x86/kvm/x86.c             | 6 ++++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index aeeb071c7688..d1c50b95bbc1 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6581,6 +6581,7 @@ branch to guests' 0x200 interrupt vector.
>  :Architectures: x86
>  :Parameters: args[0] defines which exits are disabled
>  :Returns: 0 on success, -EINVAL when args[0] contains invalid exits
> +          or if any vCPU has already been created
>  
>  Valid bits in args[0] are::
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0cf1082455df..37529c0c279d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5764,6 +5764,10 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
>  			break;
>  
> +		mutex_lock(&kvm->lock);
> +		if (kvm->created_vcpus)
> +			goto disable_exits_unlock;
> +
>  		if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
>  			kvm_can_mwait_in_guest())
>  			kvm->arch.mwait_in_guest = true;
> @@ -5774,6 +5778,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
>  			kvm->arch.cstate_in_guest = true;
>  		r = 0;
> +disable_exits_unlock:
> +		mutex_unlock(&kvm->lock);
>  		break;
>  	case KVM_CAP_MSR_PLATFORM_INFO:
>  		kvm->arch.guest_can_read_msr_platform_info = cap->args[0];
> -- 
> 2.30.2
>