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

[Jason Baron <jbaron@xxxxxxxxxx>]

On Thu, Oct 13, 2011 at 09:44:48AM -0700, Jeremy Fitzhardinge wrote:
> On 10/13/2011 03:54 AM, Peter Zijlstra wrote:
> > On Wed, 2011-10-12 at 17:51 -0700, Jeremy Fitzhardinge wrote:
> >> This is is all unnecessary complication if you're not using PV ticket
> >> locks, it also uses the jump-label machinery to use the standard
> >> "add"-based unlock in the non-PV case.
> >>
> >>         if (TICKET_SLOWPATH_FLAG &&
> >>             unlikely(static_branch(&paravirt_ticketlocks_enabled))) {
> >>                 arch_spinlock_t prev;
> >>
> >>                 prev = *lock;
> >>                 add_smp(&lock->tickets.head, TICKET_LOCK_INC);
> >>
> >>                 /* add_smp() is a full mb() */
> >>
> >>                 if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
> >>                         __ticket_unlock_slowpath(lock, prev);
> >>         } else
> >>                 __add(&lock->tickets.head, TICKET_LOCK_INC, 
> >> UNLOCK_LOCK_PREFIX); 
> > Not that I mind the jump_label usage, but didn't paravirt have an
> > existing alternative() thingy to do things like this? Or is the
> > alternative() stuff not flexible enough to express this?
> 
> Yeah, that's a good question.  There are three mechanisms with somewhat
> overlapping concerns:
> 
>   * alternative()
>   * pvops patching
>   * jump_labels
> 
> Alternative() is for low-level instruction substitution, and really only
> makes sense at the assembler level with one or two instructions.
> 
> 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 is basically an efficient way of doing conditionals
> predicated on rarely-changed booleans - so it's similar to pvops in that
> it is effectively a very ordinary C construct optimised by dynamic code
> patching.
> 

Another thing is that it can be changed at run-time...Can pvops be
adjusted at run-time as opposed to just boot-time?

thanks,

-Jason

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