Re: [PATCH 4/6] kvm tools: Add rwlock wrapper

Avi Kivity
2011-05-29T06:48:01Z

On 05/28/2011 10:45 PM, Sasha Levin wrote:
> On Sat, 2011-05-28 at 17:24 +0200, Ingo Molnar wrote:
> > * Sasha Levin<levinsasha928@gmail.com> wrote:
> >
> > > So the basic plan here is to allocate a futex(?) for each VCPU
> > > thread, and have the writer thread lock all futexes when it needs
> > > to write?
> > >
> > > If we assume we only have one writer thread, it can stay pretty
> > > simple.
> >
> > We can use an even simpler and more scalable method:
> >
> > - writers can 'stop' all other threads, by sending them a
> > threadpool signal and waiting for each thread to have completed
> > processing their current work and notifying the writer back that
> > they have stopped running.
> >
> > This means that the read-side lock is _zero instructions_, basically
> > just a barrier() to make sure the compiler does not move instructions
> > across threadpool functions (it wont).
> >
> > This method requires that we know about every worker thread - i.e.
> > no-one does a stray pthread_create() and uses data structures from
> > there. It also requires that each worker thread can 'stop' within a
> > reasonable amount of time.
>
> In this case, maybe instead of implementing it as a 'lock', we can
> implement it as a way to stop all vcpu threads from reentering the
> kernel (KVM_RUN):
>
> 1. Set a 'vcpu-stop' flag.
> 2. Signal all VCPUs to exit KVM_RUN.
> 3. VCPU threads now wait on our lock before reentering into KVM_RUN -
> the writer thread waits until waiting threads = VCPU count.
> 4. Writer thread writes, releases lock.
>
> So instead of it being a lock in MMIO, IO-ports, etc - it's a method to
> stop the entire guest which could be used during configuration updates
> (and anything else we might think of). It could also be used as a method
> for users to 'pause' the guest.

Yes, this is equivalent to the kernel's stop_machine_run(). It's a
heavyweight method but it should work just fine.

--
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