Re: [PATCH 0/5] KVM paravirt_ops implementation

Anthony Liguori wrote:

Hi Jeremy,

Jeremy Fitzhardinge wrote:

quoted

Anthony Liguori wrote:
 

quoted

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.

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.

quoted

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).

  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?

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.

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.

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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