Re: [PATCH 1/3] KVM: x86: Disallow enable KVM_CAP_X86_DISABLE_EXITS capability after vCPUs have been created

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 30, 2023, Hou Wenlong wrote:
> Disable PAUSE/MWAIT/HLT exits after vCPUs have been created is useless,
> because PAUSE/MWAIT/HLT intercepts configuration is not changed after
> vCPU created.  And two vCPUs may have inconsistent configuration if
> disable PAUSE/MWAIT/HLT exits between those two vCPUs creation. Since
> it's a per-VM capability, all vCPUs should keep same configuration, so
> disallow enable KVM_CAP_X86_DISABLE_EXITS capability after vCPUs have
> been created.
> 
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> ---
>  Documentation/virt/kvm/api.rst | 3 ++-
>  arch/x86/kvm/x86.c             | 5 +++++
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index a69e91088d76..95a683a27cf2 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7179,7 +7179,8 @@ 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
> +:Returns: 0 on success, -EINVAL when args[0] contains invalid exits or
> +	  any vCPUs have been created.
>  
>  Valid bits in args[0] are::
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2c0ff40e5345..7e97595465fc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6275,6 +6275,9 @@ 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_PAUSE)
>  			kvm->arch.pause_in_guest = true;
>  
> @@ -6296,6 +6299,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		}
>  
>  		r = 0;
> +disable_exits_unlock:

Hah!  I thought this looked familiar[*] :-)  

[*] https://lkml.kernel.org/r/DM6PR12MB35000D46146BA68EE294953BCACB9%40DM6PR12MB3500.namprd12.prod.outlook.com