Re: [PATCH v6 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:

> 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;

This looks good to me but probably not to an unprepared reader some time
later. What's going on here is:

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.

I think something like the paragraph above should be the commit
message. With this fixed,

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

-- 
Vitaly