Re: [PATCH v3 1/5] x86/kvm/hyper-v: Explicitly align hcall param for kvm_hyperv_exit

[Jon Doron <arilou@gmail.com>]

On 09/03/2020, Vitaly Kuznetsov wrote:
>Jon Doron <arilou@gmail.com> writes:
>
>> Signed-off-by: Jon Doron <arilou@gmail.com>
>> ---
>>  include/uapi/linux/kvm.h | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 4b95f9a31a2f..24b7c48ccc6f 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -197,6 +197,7 @@ struct kvm_hyperv_exit {
>>  			__u64 msg_page;
>>  		} synic;
>>  		struct {
>> +			__u32 pad;
>>  			__u64 input;
>>  			__u64 result;
>>  			__u64 params[2];
>
>This doesn't seem to be correct, __u64 get aligned at 8 byte boundary so
>implicitly you now (pre-patch) have the following:
>
>struct kvm_hyperv_exit {
>	__u32 type;
>        __u32 pad1;
>	union {
>		struct {
>			__u32 msr;
>                        __u32 pad2;
>			__u64 control;
>			__u64 evt_page;
>			__u64 msg_page;
>		} synic;
>		struct {
>			__u64 input;
>			__u64 result;
>			__u64 params[2];
>		} hcall;
>	} u;
>};
>
>and the suggestion is only to make it explicit. Adding something before
>'input' will actually break ABI.
>
>-- 
>Vitaly
>

Bah sorry guys :( not sure why this too so many iterations... will fix 
it right in next version.