[Xen-devel] Re: Performance overhead of paravirt_ops on native identified

["H. Peter Anvin" <hpa@xxxxxxxxx>]

Jeremy Fitzhardinge wrote:
> 
> We did consider something like this at the outset.  As I remember, there 
> were a few concerns:
> 
>     * There was no relocation data available in the kernel.  I played
>       around with ways to make it work, but they ended up being fairly
>       complex and brittle, with a tendency (of course) to trigger
>       binutils bugs.  Maybe that has changed.

We already do this pass (in fact, we do something like three passes of
it.)  It's basically the vmlinux.o pass.

>     * We didn't really want to implement two separate mechanisms for the
>       same thing.  Given that we wanted to inline things like
>       cli/sti/pushf/popf, we needed to have something capable of full
>       patching.  Having a separate mechanisms for patching calls is
>       harder to justify.  Now that pvops is well settled, perhaps it
>       makes sense to consider adding another more general patching
>       mechanism to avoid the indirect calls (a dynamic linker, essentially).

Full patching is understandable (although I think sometimes the code
generated was worse than out-of-line... I believe you have fixed that.)

> I won't make any great claims about the beauty of the PV_CALL* gunk, but 
> at the very least it is contained within paravirt.h.

There is still massive spillover into other code, though, at least some
of which could possibly be avoided.  I don't know.

>> (*) if patching code on SMP was cheaper, we could actually do this
>> lazily, and wouldn't have to store a list of patch sites.  I don't feel
>> brave enough to go down that route.
>>   
> The problem that the tracepoints people were trying to solve was harder, 
> where they wanted to replace an arbitrary set of instructions with some 
> other arbitrary instructions (or a call) - that would need some kind SMP 
> synchronization, both for general sanity and to keep the Intel rules happy.
> 
> In theory relinking a call should just be a single word write into the 
> instruction, but I don't know if that gets into undefined territory or 
> not.  On older P4 systems it would end up blowing away the trace cache 
> on all cpus when you write to code like that, so you'd want to be sure 
> that your references are getting resolved fairly quickly.  But its hard 
> to see how patching the offset in a call instruction would end up 
> calling something other than the old or new function.

The problem is that since the call offset field can be arbitrarily
aligned -- it could even cross page boundaries -- you still have
absolutely no SMP atomicity guarantees.  So you still have all the same
problems.  Without

        -hpa

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