For the structs containing variables with the same sizes, or already size aligned 

variables, we knew the __packed has no effect. And for these structs, it doesn't 
cause performance impact either, correct? 

But in the future, if different sized variables are added, the __packed may 
become necessary again. To prevent anyone accidently forget to add __packed 
when adding new variables to these structs, can we keep the __packed for all 
messages going through the "wire"?

It should not be a problem because anybody adding new variables should
know packed is not liked in the kernel and will take care.

If you want to be paranoid add a BUILD_BUG_ON(size(struct foo) != 42);

   Andrew