[Xen-devel] Re: [PATCH RFC V5 00/11] Paravirtualized ticketlocks

[Jason Baron <jbaron@xxxxxxxxxx>]

On Fri, Oct 14, 2011 at 12:02:43PM -0700, Jeremy Fitzhardinge wrote:
> On 10/14/2011 11:35 AM, Jason Baron wrote:
> > On Fri, Oct 14, 2011 at 10:02:35AM -0700, Jeremy Fitzhardinge wrote:
> >> On 10/14/2011 07:17 AM, Jason Baron wrote:
> >>> On Thu, Oct 13, 2011 at 09:44:48AM -0700, Jeremy Fitzhardinge wrote:
> >>>> pvops is basically a collection of ordinary _ops structures full of
> >>>> function pointers, but it has a layer of patching to help optimise it. 
> >>>> In the common case, this just replaces an indirect call with a direct
> >>>> one, but in some special cases it can inline code.  This is used for
> >>>> small, extremely performance-critical things like cli/sti, but it
> >>>> awkward to use in general because you have to specify the inlined code
> >>>> as a parameterless asm.
> >>>>
> >>> I haven't look at the pvops patching (probably should), but I was
> >>> wondering if jump labels could be used for it? Or is there something
> >>> that the pvops patching is doing that jump labels can't handle?
> >> Jump labels are essentially binary: you can use path A or path B.  pvops
> >> are multiway: there's no limit to the number of potential number of
> >> paravirtualized hypervisor implementations.  At the moment we have 4:
> >> native, Xen, KVM and lguest.
> >>
> > Yes, they are binary using the static_branch() interface. But in
> > general, the asm goto() construct, allows branching to any number of
> > labels. I have implemented the boolean static_branch() b/c it seems like
> > the most common interface for jump labels, but I imagine we will
> > introduce new interfaces as time goes on. You could of course nest
> > static_branch() calls, although I can't say I've tried it.
> 
> At the moment we're using pvops to optimise things like:
> 
>       (*pv_mmu_ops.set_pte)(...);
> 
> To do that with some kind of multiway jump label thing, then that would
> need to expand out to something akin to:
> 
>       if (static_branch(is_xen))
>               xen_set_pte(...);
>       else if (static_branch(is_kvm))
>               kvm_set_pte(...);
>       else if (static_branch(is_lguest))
>               lguest_set_pte(...);
>       else
>               native_set_pte(...);
> 
> or something similar with an actual jump table.  But I don't see how it
> offers much scope for improvement.
> 
> If there were something like:
> 
>       STATIC_INDIRECT_CALL(&pv_mmu_ops.set_pte)(...);
> 
> where the apparently indirect call is actually patched to be a direct
> call, then that would offer a large subset of what we do with pvops.
> 
> However, to completely replace pvops patching, the static branch / jump
> label mechanism would also need to work in assembler code, and be
> capable of actually patching callsites with instructions rather than
> just calls (sti/cli/pushf/popf being the most important).
> 
> We also keep track of the live registers at the callsite, and compare
> that to what registers the target functions will clobber in order to
> optimise the amount of register save/restore is needed.  And as a result
> we have some pvops functions with non-standard calling conventions to
> minimise save/restores on critical paths.
> 
> > We could have an interface, that allowed static branch(), to specifiy an
> > arbitrary number of no-ops such that call-site itself could look anyway
> > we want, if we don't know the bias at compile time. This, of course
> > means potentially greater than 1 no-op in the fast path. I assume the
> > pvops can have greater than 1 no-op in the fast path. Or is there a
> > better solution here?
> 
> See above.  But pvops patching is pretty well tuned for its job.
> 
> However, I definitely think its worth investigating some way to reduce
> the number of patching mechanisms, and if pvops patching doesn't stretch
> static jumps in unnatural ways, then perhaps that's the way to go.
> 
> Thanks,
>     J

ok, as things are now, I don't think jump labels are well suited for
replacing indirect calls. They could be used to have a single no-op that
is replaced with a jmp to the proper direct call...but at that point
you've taken an extra jump. That doesn't make sense to me.

Jump labels are well suited as mentioned for if/else type control flow,
while the indirect call table, at least to me, seems like a bit of a
different use-case...

Thanks,

-Jason

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