Re: [PATCH v2 11/49] KVM: x86: Disallow KVM_CAP_X86_DISABLE_EXITS after vCPU creation

[Xiaoyao Li]

On 5/18/2024 1:38 AM, Sean Christopherson wrote:
> Reject KVM_CAP_X86_DISABLE_EXITS if vCPUs have been created, as disabling
> PAUSE/MWAIT/HLT exits after vCPUs have been created is broken and useless,
> e.g. except for PAUSE on SVM, the relevant intercepts aren't updated after
> vCPU creation.  vCPUs may also end up with an inconsistent configuration
> if exits are disabled between creation of multiple vCPUs.
>
> Cc: Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
> Link: https://lore.kernel.org/all/9227068821b275ac547eb2ede09ec65d2281fe07.1680179693.git.houwenlong.hwl@xxxxxxxxxxxx
> Link: https://lore.kernel.org/all/20230121020738.2973-2-kechenl@xxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>

> ---
>   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 6ab8b5b7c64e..884846282d06 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7645,6 +7645,7 @@ 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 vCPUs have already been created
>   
>   Valid bits in args[0] are::
>   
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bb34891d2f0a..4cb0c150a2f8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6568,6 +6568,10 @@ 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_PAUSE)
>   			kvm->arch.pause_in_guest = true;
>   
> @@ -6589,6 +6593,8 @@ 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];