Andi Kleen wrote:

quoted

so it would be  
nice to use one of the other serializing instructions in this case.
    

You would first need to find one that works in ring 3. On x86-64 it is 
used in the gettimeoday vsyscall in ring 3 to synchronize the TSC and 
afaik John was about to implement that for i386 too.
  

Well, that's really usermode code, so I don't think we'd necessarily 
touch it at all.  It's not the same problem as the (single, at the 
moment) ring 0 use.

BTW another issue that I haven't checked but we will need to make
this also an alternative() for another case - it is faily important
to patch it out on Intel systems with a synchronized TSC where it is
fairly expensive. That is also not done yet on i386, but will be 
likely once vsyscall gettimeofday is implemented.

So basically you would need double patching. Ugly.
  

Yeah.  I guess the cleanest way to do that is do the paravirt 
substitution, and then nop it out later if it isn't needed in the vsyscall.

I would recommend to keep it out of para ops.

It's hardly a big deal either way.  There's only one in-kernel use of it.

    J