Re: [PATCH 0/5] KVM paravirt_ops implementation

Jeremy Fitzhardinge wrote:

Anthony Liguori wrote:
  

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Hi Jeremy,

Jeremy Fitzhardinge wrote:
    

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Anthony Liguori wrote:
 
      

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1) Not really sure what is needed for CONFIG_PREEMPT support.  I'm not
sure which paravirt_ops calls are actually re-entrant.
    
        

I'm not sure that has specifically come up.  The main issue is whether a
particular call can be preempted and whether that matters.  I guess the
calls which affect a particular CPU's state will generally be called in
a non-preemptable context, but I guess we can't assume that; the best
approach is to assume that each call be atomic with respect to
preemption.
  
      

So each call would need to disable preemption?  I'm not sure that
makes a whole lot of sense for something like CR reads/writes.  In
fact, without passing in a cpu parameter, I'm pretty sure that those
operations *have* to require preemption to be disabled.
    

Yeah, its a little unclear to me.  If you're poking at a control
register, then one presumes you've got a specific CPU's CRx in mind. 
But in the Xen code I don't care about the preemption state for control
register updates - except for write_cr3, which never makes any sense
with preemption enabled.

  

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For something like MMU operations, preemption really doesn't have to
be disabled.
    

Unless you're batching, since the lazy_mode is inherently per-cpu state.

  

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Things like batching must be completed with preemption disabled over the
whole batch.  I check that with BUG_ON in the Xen code.
  
      

Right now the KVM batching requires preemption to be disabled for
batching.
    

I think that's probably overkill.  I had to put a few explicit
preempt_disable/enables in the Xen code, but mostly the preempt state is
reasonable for a given operation (ie, disabled for per-cpu state
updates, enabled for memory/global state updates).

  

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  I don't think that's a hard requirement though since we could pass
the current batch PA as part of the flush hypercalls.
    

PA?

  

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Things are a lot easier though if we can just assume preemption is
disabled :-)

Are you aware of any paravirt_ops calls that are probably being called
in the kernel with preemption enabled?
    

Erm, I haven't made a breakdown, but many are.  The descriptor updates
generally are, for example.  Pagetable updates could be, but are
generally done under a pagetable lock, and so are not preemptible anyway.

  

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I don't see a compelling reason to paravirtualize earlier although I
also don't see a compelling reason not too.  I noticed that VMI hooks
setup.c.  It wasn't immediately obvious why it was hooking there but
perhaps it worthwhile to have a common hook?  I suspect VMI and KVM
will have a similar model for startup.
    

Well, I was suggesting we could print the banner later rather than
forcing an earlier init.
  

Perhaps we can just print the banner before batching occurs?  Then it's 
being printed at the last possible moment.

Regards,

Anthony Liguori

The important part is that you set your pv_ops before patching occurs,
since that will bake the function calls into the rest of the kernel, and
it will ignore any further changes to the paravirt_ops structure.

I think Zach was originally thinking of initializing VMI much later
(even as a module load), but the subtleties of inveigling its way into
the kernel at that late stage got too complex.

    J

  

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