Re: [PATCH v8 1/5] x86/kvm/hyper-v: Explicitly align hcall param for kvm_hyperv_exit

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Jon Doron <arilou@gmail.com> writes:

> The problem the patch is trying to address is the fact that 'struct
> kvm_hyperv_exit' has different layout on when compiling in 32 and 64 bit
> modes.
>
> In 64-bit mode the default alignment boundary is 64 bits thus
> forcing extra gaps after 'type' and 'msr' but in 32-bit mode the
> boundary is at 32 bits thus no extra gaps.
>
> This is an issue as even when the kernel is 64 bit, the userspace using
> the interface can be both 32 and 64 bit but the same 32 bit userspace has
> to work with 32 bit kernel.
>
> The issue is fixed by forcing the 64 bit layout, this leads to ABI
> change for 32 bit builds and while we are obviously breaking '32 bit
> userspace with 32 bit kernel' case, we're fixing the '32 bit userspace
> with 64 bit kernel' one.
>
> As the interface has no (known) users and 32 bit KVM is rather baroque
> nowadays, this seems like a reasonable decision.
>
> Signed-off-by: Jon Doron <arilou@gmail.com>
> ---
>  Documentation/virt/kvm/api.rst | 2 ++
>  include/uapi/linux/kvm.h       | 2 ++
>  2 files changed, 4 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index ebd383fba939..4872c47bbcff 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -5025,9 +5025,11 @@ EOI was received.
>    #define KVM_EXIT_HYPERV_SYNIC          1
>    #define KVM_EXIT_HYPERV_HCALL          2
>  			__u32 type;
> +			__u32 pad1;
>  			union {
>  				struct {
>  					__u32 msr;
> +					__u32 pad2;
>  					__u64 control;
>  					__u64 evt_page;
>  					__u64 msg_page;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 4b95f9a31a2f..7ee0ddc4c457 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -189,9 +189,11 @@ struct kvm_hyperv_exit {
>  #define KVM_EXIT_HYPERV_SYNIC          1
>  #define KVM_EXIT_HYPERV_HCALL          2
>  	__u32 type;
> +	__u32 pad1;
>  	union {
>  		struct {
>  			__u32 msr;
> +			__u32 pad2;
>  			__u64 control;
>  			__u64 evt_page;
>  			__u64 msg_page;

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly