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

[Jason Baron <jbaron@xxxxxxxxxx>]

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.

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?

> As I said, pvops patching is very general since it allows a particular
> op site to be either patched with a direct call/jump to the target code,
> or have code inserted inline at the site.  In fact, it probably wouldn't
> take very much to allow it to implement jump labels.
> 
> And the pvops patching mechanism is certainly general to any *ops style
> structure which is initialized once (or rarely) and could be optimised. 
> LSM, perhaps?
> 
> >> 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?
> 
> No.  In general that wouldn't really make sense, because once you've
> booted on one hypervisor you're stuck there (though hypothetically you
> could consider migration between machines with different hypervisors). 
> In some cases it might make sense though, such as switching on PV
> ticketlocks if the host system becomes overcommitted, but leaving the
> native ticketlocks enabled if not.
> 
>     J

A nice featuer of jump labels, is that it allows the various branches
(currently we only support 2), to be written in c code (as opposed to asm),
which means you can write your code as you normally would and access any
parameters as you normally would - hopefully, making the code pretty
readable as well.

I hope this better clarifies the use-cases for the various mechanisms.

Thanks,

-Jason

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