Re: [PATCH for-5.18] KVM: fix bad user ABI for KVM_EXIT_SYSTEM_EVENT

[Sean Christopherson <seanjc@google.com>]

On Fri, Apr 22, 2022, Paolo Bonzini wrote:
> For compatibility with userspace that was using the flags field,
> a union overlaps flags with data[0].

I think "compatibility" is slightly misleading, e.g. the offset of the field is
changing for 32-bit userspace. 

  To avoid breaking compilation of userspace that was using the flags
  field, provide a userspace-only union to overlap flags with data[0].

> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 91a6fe4e02c0..f903ab0c8d7a 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -445,7 +445,11 @@ struct kvm_run {
>  #define KVM_SYSTEM_EVENT_RESET          2
>  #define KVM_SYSTEM_EVENT_CRASH          3
>  			__u32 type;
> -			__u64 flags;
> +			__u32 ndata;
> +			union {
> +				__u64 flags;

As alluded to above, what about wrapping flags in 

#ifndef __KERNEL__
				__u64 flags;
#endif

so that KVM doesn't try to use flags?

> +				__u64 data[16];
> +			};
>  		} system_event;
>  		/* KVM_EXIT_S390_STSI */
>  		struct {