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

[Xiaoyao Li]

On 3/30/2023 8:35 PM, 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.  

PAUSE intercepts can be updated via 
SET_CPUID->vmx_vcpu_after_set_cpuid() after vCPU is created.

Aside it, this patch looks good to me.

> 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@xxxxxxxxxxxx>
> ---
>   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:
> +		mutex_unlock(&kvm->lock);
>   		break;
>   	case KVM_CAP_MSR_PLATFORM_INFO:
>   		kvm->arch.guest_can_read_msr_platform_info = cap->args[0];
>
> base-commit: 27d6845d258b67f4eb3debe062b7dacc67e0c393