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

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

Jeremy Fitzhardinge wrote:
> 
> So, what's the fix?
> 
> Paravirt patching turns all the pvops calls into direct calls, so
> _spin_lock etc do end up having direct calls.  For example, the compiler
> generated code for paravirtualized _spin_lock is:
> 
> <_spin_lock+0>:               mov    %gs:0xb4c8,%rax
> <_spin_lock+9>:               incl   0xffffffffffffe044(%rax)
> <_spin_lock+15>:      callq  *0xffffffff805a5b30
> <_spin_lock+22>:      retq
> 
> The indirect call will get patched to:
> <_spin_lock+0>:               mov    %gs:0xb4c8,%rax
> <_spin_lock+9>:               incl   0xffffffffffffe044(%rax)
> <_spin_lock+15>:      callq <__ticket_spin_lock>
> <_spin_lock+20>:      nop; nop                /* or whatever 2-byte nop */
> <_spin_lock+22>:      retq
> 
> One possibility is to inline _spin_lock, etc, when building an
> optimised kernel (ie, when there's no spinlock/preempt
> instrumentation/debugging enabled).  That will remove the outer
> call/return pair, returning the instruction stream to a single
> call/return, which will presumably execute the same as the non-pvops
> case.  The downsides arel 1) it will replicate the
> preempt_disable/enable code at eack lock/unlock callsite; this code is
> fairly small, but not nothing; and 2) the spinlock definitions are
> already a very heavily tangled mass of #ifdefs and other preprocessor
> magic, and making any changes will be non-trivial.
> 

The other obvious option, it would seem to me, would be to eliminate the
*inner* call/return pair, i.e. merging the _spin_lock setup code in with
the internals of each available implementation (in the case above,
__ticket_spin_lock).  This is effectively what happens on native.  The
one problem with that is that every callsite now becomes a patching target.

That brings me to a somewhat half-arsed thought I have been walking
around with for a while.

Consider a paravirt -- or for that matter any other call which is
runtime-static; this isn't just limited to paravirt -- function which
looks to the C compiler just like any other external function -- no
indirection.  We can point it by default to a function which is really
just an indirect jump to the appropriate handler, that handles the
prepatching case.  However, a linktime pass over vmlinux.o can find all
the points where this function is called, and turn it into a list of
patch sites(*).  The advantages are:

1. [minor] no additional nop padding due to indirect function calls.
2. [major] no need for a ton of wrapper macros manifest in the code.

paravirt_ops that turn into pure inline code in the native case is
obviously another ball of wax entirely; there inline assembly wrappers
are simply unavoidable.

        -hpa

(*) 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.


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